I normally like to keep guides concise, but there's so much to cover for getting a more difficult achievement like this. To get Mare Nostrum, you need to own the entire coastline of the Mediterranean and Black Sea, and form the Roman Empire. To form the Roman Empire, you need to be either the Christian or Pagan religion group, and own specific regions and provinces (more details on this can be found on the EU4 wiki site[www.eu4wiki.com]).


It's not easy to fill out the requirements, and you'll have to walk a fine line managing your economy, overextension, aggressive expansion, armies, and diplomacy. If you have any questions, please leave them in the comment section, and I'll do my best to answer in a timely fashion.

I'm going to make the case that the easiest way to form Rome will be to start as Aragon. It's easy to get the entire Iberian Peninsula and wreck the Ottomans early on, and you can get several other rare achievements along the way.

If you'd like to watch a video before reading more, I explain some of the concepts here:

I'll add screenshots and/or videos depending on the questions in the comments section, since how clearly the information is presented can always be improved on. You'll notice some of my screenshots don't show Aragon or Spain - and there's an explanation for that. My save files were wiped out as of the Third Rome DLC, so I had to start new games to get screenshots.

The first few years will be identical to the opening moves for getting the Consulate of the Sea Achievement. You can find more details on my guide here.

This is unfortunately dependent on luck. The Ottomans may not attack Albania first, or they may attack Albania too soon, or Venice might attack Albania before the Ottomans do. At least you'll know within the first 5 or 10 years of the game whether you can execute that strategy. Once you have Biga, Hudavendigar, Kocaeli, and have vassalized Byzantium, you're off to a good start.

I picked Aragon because it has several advantages:

With Aragon's powerful navy and the Ottomans' irrational propensity to commit 100% of their forces to fighting Albania, you can easily wreck the Ottomans early on and pick up centers of trade.

When the Iberian wedding event fires, the human player always leads the PU. That means that you can have Castile, Naples, and Byzantium as subjects.

Aragonese ideas really emphasize trade and naval force limit (which can be used to protect trade and boost naval tradition). There's also a stability cost reduction and extra autonomy change, which means you will get fewer rebellions as you steamroll across Europe, North Africa, and the Middle East. Even when you do get rebellions, those cheaper mercenaries make them easy to handle without running low on manpower.

Being Catholic also means access to the Holy See, which can be used for all sorts of goodies like extra stability, tax income, or diplomatic reputation. The bigger you get, the more bishops you will have, which means more Papal Influence over time and more goodies.

When you start the game, you have two promoted cultures, Catalan and Sicilian, along with your primary culture Aragonese. You will start out expanding into Anatolia, and with Turkish being an non-accepted culture and the religion not being Catholic there will be a lot of unrest. One way to mitigate this is to make Turkish an accepted culture. That by itself will reduce unrest, and make it easier to convert the provinces to Catholic. Unfortunately, both promoted culture slots are already taken up, and you don't want to demote Sicilian or Catalan since that increases unrest and you lose the manpower and income bonuses that come with promoted culture.

I'd suggest you immediately culture shift the three Aragonese provinces to Catalan. Aragonese starts out with only 16% of your development, compare to 43% for Catalan. You want to do this right away, because you can only culture shift to a culture that is at least 50% of your development, and if you wait until you make the Turkish provinces full cores you will not be able to easily push Catalan above the 50% threshold.

Once the provinces are Catalan, that should be about 59% of your development, and you can switch Catalan to your primary culture. Now Aragonese and Sicilian will be the two promoted cultures. You should demote Aragonese. Normally demoting a culture increases unrest, but since 0% of your provinces are Argonese you won't get any unrest.

Once the Turkish provinces are full cores, promote Turkish.

All told, this will cost about 456 diplomatic points:
Shift Pirineo: 52
Shift Zaragoza: 127
Shift Teruel: 67
Culture Shift Catalan: 100
Demote Aragonese: 10
Promote Turkish: 100

You could probably still do this while leaving Pirineo Aragonese if you really feel you can't spare 52 diplo points, and the unrest in that one province will be manageable. I know 456 diplo points sounds like a lot but that is less than what you would normally spend on a tech level and it will really help with unrest in Anatolia.

Some of you have probably experienced personal unions at some point or another. Most likely your first one happened by accident. They seem nearly impossible to get unless you save-scum, and if you really want to save-scum I won't judge. However, if you want a way to intentionally get personal unions without resorting to exploits, I've got a strategy worked out. Personal Unions are great because it allows you to quickly pick up vast tracks of territory without any aggressive expansion, and within 50 years you can begin integrating the junior partner in a PU.

You'll want your first Idea Group to be Diplomatic Ideas. The bonus for completing this idea group has "lowered impact on stability from diplomatic actions." Basically any diplomatic action that could impact stability (like breaking royal ties, or breaking a truce) will lower your stability. With this idea group, you get less of a penalty. So much so that to break a royal marriage will only cost you a single prestige point. That's all. It's practically free.

Once you have the idea group filled out, you can keep an eye on the Disputed Succession Tab in the top of the screen, and send off royal marriages to whoever is at risk. The worst case scenario is an heir is born, and you simply royal divorce them. This will cause you the target country's opinion of you to drop by 75, but honestly if an heir was born there is plenty of time for the target country to forget all about that the next time they are at risk of falling into a personal union.

If you are really lucky, the country you royal married dies with no heir, and you inherit the country. You might have to fight with a rival over the succession, but hopefully you're big enough that you can beat whoever is fighting you.

Alternatively, if your prestige is too low, the target country is too big, or the target country happens to be at war when the ruler dies, the target country will get a new ruler of your same dynasty. This is not as bad as it sounds. If the new ruler either has a weak claim, or has no heir, you can fight a Claim Throne CB. If you get 84% war score, you can force a PU on the target country. Normally to do this you would need to break royal ties and quickly break the truce before an heir is born, otherwise you lose the CB. Normally this would cost you 6 stability points (1 for breaking the royal marriage, and 5 for breaking the truce). Not so with Diplomatic Ideas. You will only lose 3 stability for breaking the truce. Three stability still sounds like a lot, but depending on stability cost modifiers and Papal Influence, it's very manageable. That means that you can attack a former ally within a mere 61 days (the time it takes to send a diplomat 3 times to break the alliance, send a divorce, and declare war). If the target's forts were mothballed, there will barely be any notice before your attack.

Eventually you can form Spain diplomatically. It won't cost you any diplo points, and it automatically upgrades you to Empire government rank. All you need is Castile as a subject and 100 legitimacy, so there's no reason not to do it.

I would recommend you keep the Aragonese Ideas once you do it. Spain's ideas are more geared towards colonization, and don't sync well with a strategy aimed at re-forming the Roman Empire. The idea of Spain having Aragonese Ideas with Catalan being the primary culture sounds odd, but it works best for this strategy.

That brings me to another achievement you can pick up. Mare Nostrum is difficult to achieve, and investing time in an entire campaign while not getting an achievement can be frustrating. Fortunately you can also go for some other achievements, making the game worthwhile even if you don't form Rome. One of these involves getting elected as Emperor of the HRE as Spain.

This should not be too difficult if you stack your diplomatic reputation and get friendly with the electors. The Diplomacy Ideas gives you +2 diplo rep. Getting the right advisor can give you +1 more. Investing 50 points in the Holy See can get you a Papal Legate, which is +1 more. Eventually you'll want to get the Influence Ideas Group, which gives you +2 more. If that isn't enough, you can easily send royal marriages and alliances off to electors. Even if you go over your diplomacy limit you only need to get elected once to get the achievement and you can easily break those ties once you get elected (another reason why Diplomacy Ideas are nice).

Passing reforms should not be too difficult. You can wait until most princes are in favor of a reform, quickly add 50 of your own provinces to the empire, then pass a reform. You should have no trouble getting 200 or more European provinces, which is enough for 4 reforms.

A word of warning: you do not want to pass too many reforms. I'd say the first 4 are enough. If you disallow internal HRE wars, you'll have a more difficult time conquering the provinces you need to form the Roman Empire.

You also don't want to become a member of the HRE yourself. It will downgrade your government rank to Duchy, which will make it impossible for you to get the Absolutely Achievement, which is in the next section.

You can pick up the Absolutely achievement in this play through, which you get by accumulating 100 Absolutism. Honestly it will be easier to get this achievement as a few other nations (like France, Prussia, or Sweden), but if you don't already have the achievement then you might as well get it in this campaign.

You can only accumulate Absolutism during the Age of Absolutism, and there is a hard limit on how much you can get. For a monarchy the base is 65. As Spain, you can increase this base, just barely. Here's the breakdown:

Base: 65
Empire Rank: +5
100% Religious Unity: +5
Great Power: +5
Golden Era: +5
Despotic Monarchy: +5
100 Legitimacy: +10

You get Empire Rank just by forming Spain, and as long as you do not make yourself a member of the HRE you will keep this rank.

You can get 100% religious unity either by having amazing missionary strength or targeting the Humanist Idea group. I'd suggest the Humanist Ideas, since it gives a +25% buff (which makes having at least 100% far more reliable), and has some other great bonuses, such as lower unrest, increased religious tolerance, and extra accepted cultures.

Becoming a Great Power should not be difficult. In fact, I'd be surprised if you were not the #1 Great Power by the time the Age of Absolutism begins.

You only get one Golden Era, so make sure you save it for the Age of Absolutism. If you start your Golden Era sooner than this, you won't be able to get the Absolutely Achievement as Spain. Every Era has objectives, and you need to fill out 3 objectives to start a Golden Era. I'd recommend you get 5 promoted cultures (another reason why Humanist Ideas are great), build 5 universities (very easy to do), and get a force limit of over 200. The Age of Absolutism usually starts around 1610, and you should be big enough by then to have a force limit of 200. I'd recommend you go for the Quantity Ideas Group and build some regimental camps just to be on the safe side.

I want to really emphasize that you only get one Golden Era per game, because it's very easy to miss. You can access the Era Screen at the top of the screen (screenshot below).

Once there, you can see what the requirements are, what bonuses you get once you accumulate 800 Splendor, and you'll notice near the top a button that says "Start Golden Era". This button can only be used once, and there is a message that says that when you hover the mouse over it, but like I said it's easy to miss and I'm sure 99% of EU4 players missed it the first time they clicked it.

Once your golden age has begun, it lasts for 50 years (so I'd recommend waiting until you've already got your Absolutism pretty high, since the Golden Age only provides a +5 boost and you need to reach 100 within that 50 year time frame).

Aragon starts as a Feudal Monarchy, but changing to Despotic Monarchy only costs 100 Admin Points.

Having 100 Legitimacy is not hard. There is an interaction with the Holy See to spend Papal Influence on sinful indulgences, which somehow increases legitimacy. You can also spend 100 Military Monarch Points to boost legitimacy by 10. If that wasn't enough, it usually increases on its own.

If you do all that, you will increase your potential Absolutism to 100, but that does not mean your Absolutism will automatically be 100. There are several ways to boost it. There's an ability during the Age that gives you +1 Absolutism per year. You can also get it by using harsh treatment on rebellions, and by decreasing autonomy (both methods can be used in tandem with each other assuming you can spare the monarch points).

Another way to boost Absolutism is to take the Strengthen Government option. Normally this increases legitimacy by 10, but during the Age of Absolutism it also increases Absolutism by 2. However, you can only Strengthen Government if your legitimacy is currently less than 100. There is an easy way around this, and once again those Diplomatic Ideas come into play. Every time you start a royal marriage, you lose 5 legitimacy, which enables you to Strengthen Government again. Since you'll only lose 1 prestige point for breaking off a royal marriage, you can do this just about as many times as there are countries willing to agree to a royal marriage. You can really only do this until you've already got 90 Absolutism, since having less than 100 legitimacy will lower your maximum Absolutism to 90. I'd recommend that once you get to 90 Absolutism you quickly lower autonomy in recently conquered provinces and use harsh treatment on rebellions to boost Absolutism up to 100.

Once you get the bonuses listed above, the Government Screen should show you a maximum Absolutism of 100, and hovering your mouse over that number will break down how the maximum is calculated.

At some point you will want the Influence Ideas Group. There are several reasons why this group is amazing, especially when combined with the PU Machine.

Integrated Elites reduce annexation cost by 25%, which will save you a lot of diplo points over the course of the game.

State Propaganda will reduce Aggressive Expansion by 20%, which will help you grow without triggering coalitions.

The increased diplomatic reputation will help you get elected emperor of the HRE.

Postal Service gives you an extra diplo relation, which again helps with using royal marriages to get PUs.

The Bonus of -50% unjustified demands is amazing. An unjustified demand is any province you take in the peace deal that you don't have a claim on. For example, if you fight the Papal State and have a claim on Ancona, but then also take Urbino in the peace deal, it will cost you diplo points (about 3 per development). If you have -50% unjustified demands, it will only cost you 1.5 diplo points per development.

While all this is going on, you might get a terrible 1/1/1 heir. Or worse. There is an easy way around this. In a more recent patch you have the ability to disinherit an heir at the cost of 50 prestige. Normally you don't want prestige to go too low, especially with personal unions which you are at risk of losing with negative prestige. However, there are several ways around this.

Typically in a peace deal you will get prestige from various peace deal options. If you can't take too many provinces in a peace deal (either due to AE, overextension, or limited monarch points), you can instead spend war score to force the target nation to end alliances or end rivalries. This is a very easy way to quickly build up prestige. Once your prestige is near 100, simply disinherit the terrible heir, and you'll probably get a better one.

Alternatively if you have an amazing 6/6/6 heir you can disinherit your ruler at the same cost of 50 prestige.

Keep in mind that you can only do this if you have a monarchy, so don't go switching you government type to Republic. In fact, a Republic would lower your total Absolutism (locking you out of the Absolutely achievement), and prevent you from getting PUs. In short, a Republican form of government would be terrible for this strategy. Don't do it.

The more money you have, the easier this will be. It took me a while to really understand the economy, but as you execute this strategy money should not be much of a problem. You'll have some really good centers of trade, including the end nodes of Genoa and Venice. All that extra money will make mercenaries more affordable, which helps you conserve manpower and keep the wars going.

I'd recommend you send your first two merchants to collect trade income in Genoa and Venice. Then send any additional merchants you get to transfer trade power in the nodes immediately downstream, such as Ragusa and Tunis. You could get extra merchants with certain Idea Groups, with Colonial Nations that have at least 10 provinces, or with Trade Companies, but I wouldn't worry too much about focusing on those. You'll get so much trade income anyway.

Once you conquer a center of trade, you should make that region a State, then assign the actual center of trade to the Merchant Guild. This will increase their influence a bit, to make it easier to ask for Diplo Points or a financial contribution occasionally.

You should also build workshops and churches anywhere where it will boost income by at least .10 ducats per month. When the game first starts you'll still come out ahead even if it's only an increase of .03 ducats per month (even longer if you use the exploit that lets the game go beyond 1821), but .10 is an easier rule of thumb and gets you a return on your investment faster. I would even say taking out loans is worth it to quickly build these buildings since even with interest you will come out ahead. When you can build manufactories, any time you get at least a .30 ducats per month increase, you should build those. Another advantage to building workshops and manufactories is that will increase your trade income.

If at some point you have more monarch points than you can use (for example you have 999 diplo points and a tech penalty of 130% for being 13 years ahead of time), you can develop provinces (assuming you have the Common Sense DLC). If you don't have the DLC, you can still spend Diplo Points to increase mercantilism or culture shift (which is cheaper with Aragonese Ideas). Accepted cultures get more production and tax income.

This also took me a long time to understand, but I'll try to summarize it.

There is a concept called combat width, and this gradually increases over time based on military technology (depending a bit on terrain). If the combat with is 11, then in a battle your army will automatically arrange 11 units in the front row, and 11 units in the back row, unless you have fewer than 22 units. If you have more than 22 units (especially lots of infantry), then your units will automatically arrange themselves so that regiments with more men are in front.

As a rule of thumb in any battle you want 1.5 times your combat width in infantry. That way if any regiments are half strength you will likely have enough full strength regiments to fill up the front row. You also want 4 - 6 cavalry (assuming you have Western Tech like Aragon does). I find this generally does not change throughout the game, since cavalry become less important over time. They're great for flanking, which will damage the enemy’s combat row on either end. But otherwise not good for much else when you consider what they cost. You'll also want 1 times your combat width in artillery. Assuming the front row has enough regiments to fill out, the artillery regiments will automatically position themselves in the back row. They will not take any damage back there, but can still deal damage to the enemy's front row.

In the example where the combat width is 11, you would want 17 infantry, 6 cavalry, and 11 artillery in a battle. That's a total of 34 regiments. Normally if you had a stack of 34 regiments walking around in the mid-game you would suffer a ton of attrition. The solution is simple. Always keep your artillery stacks separate from your infantry/cavalry stacks. This will significantly reduce attrition, and it enables you to maximize the siege bonus since your entire siege stack will be nothing but artillery. When you are sieging an enemy fort, make sure that your infantry/cavalry army stack is immediately adjacent, and make sure they can walk onto the fort in a moment's notice if an enemy army attempts to relieve the siege.

You should also hire some mercenaries for your infantry units. They will not drain your manpower pool when damaged. Do not ever hire mercenary cavalry or artillery, since they are extremely expensive and will probably conserve even less manpower since they take less damage than infantry.

If you're filling out a military Idea Group, only spend the monarch points if you are ahead of time in military technology. There are significant penalties to being 1 or 2 tech levels behind in military technology. Don't worry, you'll eventually fill out your Idea Group no matter what.

I think the main limiting factor in my growth is usually aggressive expansion, and this is a concept where I could probably still learn more on how to manage.

I will say that whenever you make a peace deal, keep an eye on the coalition potential in the peace deal screen. I have screenshots of this in my Consulate of the Sea Achievement Guide. In summary, the more development you take in a peace deal, the more AE you will accrue. Generally countries of the same religion as the country you're taking land from will accumulate more AE against you, and countries that are geographically close to the provinces you are taking will get more AE. Once a country has more than 50 AE against you, they have the potential to join a coalition. They won't always join a coalition, since the AI factors in things like your army strength versus the relative strength of members already in the coalition.

You have a lot of time to conquer the required provinces, so if AE is an issue then I'd recommend you slow down on taking provinces, or in peace deals force nations to release one-province-minors who you can subsequently diplo vassalize and annex 10 years later. That will cost diplo points, but it won't cost any admin points.

Assuming AE is not an issue, then another limiting factor will be overextension. The more development in non-cored provinces you have, the more overextension you will get. This gets easier over time since Admin Tech levels 17, 23, & 27 will increase your Administrative Efficiency. Having Absolutism will also increase Admin Efficiency. The more Admin Efficiency you have, the less admin points it costs to core provinces, the less diplo points it costs to annex subjects, and the less overextension you get for conquering provinces. By the late game it can get really easy to conquer vast tracts of land (especially if you don't have the Common Sense DLC, since nobody can spend monarch points increasing development).

You can see how much a peace deal will add to overextension in the peace deal screen. As a rule of thumb, you do not want to go over 100%. If AE is truly not an issue and you've gotten big enough to keep winning wars easily, then I would recommend in the peace deal you take as many provinces as you can up to about 100% overextension. Then immediately start coring them, and immediately attack the next country. If you get the next country carpet sieged before your previous batch of provinces is cored, simply wait around looting provinces. As soon as your first batch of provinces are cored, peace out the second nation up to about 100% overextension, start coring those provinces, and just repeat over and over again.

Like I said at the beginning of this guide, Mare Nostrum is a very difficult achievement to get, so even if it doesn't work out I'm hoping you can pick up some other rare achievements along the way. Here are some tips to keep in mind while playing:

In the beginning, ally France and rival the Ottomans. This will effectively prevent a Franco-Ottoman alliance from ever happening, and a Franco-Ottoman alliance is the last thing you'd ever want for trying to form the Roman Empire.

Avoid fighting Castile. It will be a waste of time since you'll eventually get all of Castile in the Iberian Wedding. Avoiding a fight should not be too difficult since your big ally France has your back.

Avoid diplo annexing Naples for as long as you can. Occasionally, if your ruler dies while your stability is 3, you completely inherit subject nations for free. If that happens, you'll save a ton of diplo points. If, however, you really need the diplo slot freed up or you are on the verge of filling out the requirements to reform Rome, then go ahead and diplo annex Naples.

Diplomatic Ideas are great. The increased diplo rep means that you're more likely to be elected emperor of the HRE. The extra diplomat makes fabricating claims easier. The buff to improving relations speeds up diplo vassalizing small nations. There's a 10% reduction in diplo technology, which can really accumulate over the course of the game. Then there's the lowered impact on stability from diplo actions, which combined with royal marriages makes intentionally getting PUs a possibility and enables you to lower legitimacy any time you want to keep spending military MP on increasing Absolutism.

Eventually get Influence Ideas. There's more diplo rep, which you will probably need to get elected emperor of the HRE. It will reduce your aggressive expansion and also make diplo annexing subjects easier. The 50% cheaper unjustified demands will save you a lot of diplo points.

Go full Humanist. If you're constantly conquering, you will deal with a lot of unrest, but to the extent that Humanist Ideas directly reduce unrest, increase tolerance of heretics and heathens, and increase religious unity, every rebellion that never happens is manpower you can conserve for fighting the next war. Getting those extra promoted cultures is key to starting your Golden Age during the Age of Absolutism.

Get Quantity Ideas. You'll also want a force limit of over 200 to help start that Golden Age during Absolutism, and when you eventually fight big countries like France, Austria, Hungary, and the Ottomans that extra manpower will really help. You'll also get cheaper regiment upkeep.

Beyond the first 4 idea groups I listed, use your best judgement. Having Quality Ideas along with Quantity will really help in those wars later in the game, but it really depends on the political situation of the game you're playing. Maybe by the mid-game if you've conquered most of the Catholic provinces you need, and the remaining provinces are Protestant, Reformed, Orthodox, or Sunni, you could go for Religion Ideas for the Deus Vault CB.

Stay Catholic, and avoid conquering Rome for as long as possible. The bonuses you can get from the Holy See can be really useful. The bigger you get, the more bishops you will get, which will increase the rate at which you accumulate Papal Influence.

The most important step of all is actually forming the Roman Empire. Once you conquer Rome, the decisions to Re-Form the Roman Empire will appear in the National Decisions Screen. Here are before and after screenshots as reference.

Before:

After: