I am an avid EU IV player, and I managed to conquer the world a few times in this game.
Many claim EU IV is difficult and a constant challenge. I personally disagree. Even in EU III I never had problems with cascading alliances that other players struggled with. Not only that I actively used cascading alliances to my advantage, it was a feature that allowed me to conquer more and easier that it would otherwise be.
The biggest problem with EU series of games is that most players play it wrong. They do not understand basics of diplomacy and this consistently make wrong decisions that then come back to bite them. Afterwards they consistently fail to understand what went wrong and go to forums to complain the game is hard.
To help such players I will explain how to play EU IV, particularly how to run diplomacy. Other aspects of the game such as trade or battle mechanics are sufficiently covered elsewhere so I will not cover them here.
To begin with alliances and diplomacy in EU IV is run on principle of 'enemy of my enemy is my friend'. This is a fancy phrase that people who like famous quotes probably heard of. However here is not just words, it literary means what it says.
Countries ally or don't ally with you not because they like or do not like you, but because they either have or do not have common enemies.
What that means is that countries first decided who will be their enemies. The rival system where you have to select 3 rivals is not just a feature creep gimmick, its core mechanic around which diplomacy in this game works. You selection rivals indicates to the AI where you fit in the complex diplomatic network of interlocking alliances. AI then makes its decisions about you based on these selections.
The most visible outcome of picking rivals is the fact that countries you selected as rivals begin to hate you. That sounds like a bad thing, but benefits do outweigh the drawbacks. I am not talking about the rewards you get for defeating the rivals, but about the fact that countries that selected the same rivals as you did, begin to see you much more favorably and more than willing to form an alliance with you. Bonus to opinion may look insignificant at glance but real benefit is much larger: not only they are much more likely to form an alliance with you when you have the same rival, but they are also much more willing to join your wars against this rival. Understanding this fact suddenly turns alliances from unpredictable headache where people constantly ask on forums things like 'why ally betrayed them' or 'why they would not join their war' into a predictable and efficient tool of expansion.
So why this works exactly. To answer that I need to explain how AI in this game thinks and makes its decisions. As I already mentioned, other countries form and keep their alliances with you not because they are helpful, or friendly or the like, but entirely because they want to fight against the same enemy you have. AI here is as much of a world conquer as some of the most seasoned players like DDRJake or PrawnStar. First they decide where they want to expand (what provinces they want). Afterwards they designate countries that hold these provinces as rivals. Only after that they seek out those who will fight these countries together with them (those selected these countries as their rivals). That is how they think, understand that, follow my advise and everything you do will start fall into place like jigwas puzzle: AI will flood you with alliance offers, they all will join your wars and you will start playing this game as a piano virtuouso.
There is a part two however. What happens when war ends. You did all of the above, brought all your allies into war, you combined armies obliterated your enemy without much trouble, you occupied everything to 100% warscore and you made peace, grabbing all that juicy land. Now you celebrate with your allies your grand victory ... erm, they all broke their alliances and rivalled you instead. Why, what did you do wrong?
To begin with there is nothing wrong with allies betraying you. Its but a normal part of the game you should be prepared to. To begin with I will explain why it happens.
As I already mentioned, AI makes alliances to fight common enemies. Ir or when you win, this enemy will either be significantly weakened, crippled or may even disappeared completely. This will make AI to reevaluate its alliances. As another famous quote goes, 'Britain does not have permanent allies, Britain has permanent interests', AI in this game also does not have permanent allies ('historical friend' parameter for certain countries is exception to that rule). AI never considered you a permanent ally to being with, it only saw you as useful ally when you had common enemies. Now that this enemy is gone, AI will re-evaluate its situation. Chances are they now border you instead of their former rival, making you an appealing target for expansion. Probability of them rivalling you is even higher if you took provinces they wanted for themselves.
However even if you avoided pissing them off by not taking land they wanted or gave them land in peace deal to keep them happy, chances are they will still rival you eventually, even if later. Reason for that is the simple fact that there are but finite direction to expand and sooner or later they will have no other enemy they can expand into.
So do not, try to keep any of your alliances. See them from the start as expedient. Plan ahead to the time when situation will change and they will betray you. While still allied to your current allies, clandestinely improve relationship with their rivals, further away from you. When time comes and your current allies will betray you, you will be immediately able to ally with their rivals and will be more than prepared to take your former allies down. Rinse and repeat ad infinite.
All of this is particularly obvious if you play in HRE as a minor nation. In an alliance of 5 OPMs against another OPM, no matter who will manage to snatch that one province, they will certainly piss the rest of his allies off and they all will rival them. When that happens, you will not ask why your allies betrayed you, as reasons will be obvious to you.
With bigger countries this simple but iron clad rule and logic will get lost among the multitude of other factors that come into play. When Castille finally betrays you after 5th joint war with France, you will probably fail to get to the bottom of the actual reasons for that sudden change of heart and will just dismiss it as some fickleness of the game or AI's heart and act like a dumped ex instead of understanding actual reason.
In EU IV no matter if a country is an OPM or a major power, the fundamentals remain the same. All major powers fundamentally think the same way as any OPM and if geopolitics change, they too will betray you.
Generally, I recommend playing in HRE to learn the game. HRE is EU IV equivalent of Ireland of Crusader Kings 2. Many OPMs without any special national ideas, mission trees or historical modifiers will allow you to experience EU IV diplomacy in its purest form. Skills you can learn by playing in HRE will make the rest of the EU IV feel almost like breeze. By the time you will border something large like France you will feel neither awe towards their blobness, nor fear of their blueness. Instead you will skillfully and effortlessly dismember it like butcher a cow carcass. You do not have to play something really challenging like East Frisia, something like Hesse, a minor power surrounded by other minor powers will do just well.
Do not play Portugal as other players suggest. Not because it's hard, but because special modifiers like historical friend with Castille do distort game rules and make you learn wrong things like trusting your allies the same way you could trust Castille as Portugal. When you play your next game and you ally will betray you, you will come to forums to complain like so many other noobs out there.
That is pretty much it. There are a few other tricks out there. For example, countries tend to not join coalitions against you if they have positive opinion about you. They get less aggressive expansion against you too if they have high opinion. Thus, have as many diplomats as you can get and send them all to improve opinion.
Countries that border you are unlikely to ally you and will not make a good ally. Even if they did not rivalled you, they sill may get border friction modifier and that will ruin their opinion and increase their chance turning hostile.
When dismantling large countries like France, avoid creating direct border with your allies as long as possible. Direct border increase chance they will break the alliance and rival you. Ideally you want to create direct border only when you annex them in final war.
Split large countries into small unconnected exclaves. That way they will not be able to merge their stacks and build a large enough army to give you hard time in the next war.
Use vassals and vassal feeding. Use royal marriages and personal unions too if opportunity presents itself. Read separate guidelines to learn how.
Finally, as counterintuitive as some may think, Diplomacy is your biggest friend and biggest enabler of conquest. Open with Diplo ideas and take Influence too as 2nd or 3rd if short on Diplo points. Military ideas are good but they are not as essential as diplomatic ones. 20% or even 100% plus discipline to your Prussian Space Marines will not do squat to 8 times larger stack of someone with 7 allies. Economic or Trade fundamentally do not matter much. Fall behind on Naval Research if you must, but do not neglect diplomacy. To paraphrase famous quote 'we always invest in diplomacy and it always pays'.
This is how you play EU IV. This is the one crucial missing piece of the puzzle that prevents you from conquering the world.









