Europa Universalis IV is a game about numbers… lots of numbers and how they interact with lots of other numbers. As a result, nothing is ever as simple or one sided as it might appear, which is one reason the game has so much re-playability. If you've been playing for a while you're likely to have stumbled onto your favourite builds and army compositions, but are they optimal?
There's a lot to talk about when it comes to your armies and we'll do our best to cover most of it here, sharing what we've found from playing the game and what others have contributed to the endless discussion.
Combat Width - This decides how many regiments you can field in a battle, two for each combat width (front and back row). This number starts low and increases as your technology progresses. You should aim to have enough units to fill at least one row where possible, with any artillery being on top of this.
Unit Pips - When it comes to choosing which unit to field, pips matter. During each combat phase offensive pips increase damage dealt, and defensive pips decrease damage taken. After artillery becomes available, defensive pips become more valuable as they effect both lines. Morale pips should take top priority here as they are used in both combat phases.
Deployment - In simple terms, the front row will fill with infantry with a small number of cavalry to the sides. Artillery will be positioned in the back row unless they outnumber the infantry and cavalry, in which case they will even out the rows by adding some to the front. Any space left over in the second row will fill first with infantry then cavalry. Any excess units are added to the reserve.
Phases - Each combat phase lasts three days and alternates between Fire and Shock and both sides roll a die at the beginning of each. This is combined with any other modifiers (leader/terrain/unit pips) but cannot equal less than zero. Each day of a combat phase, the game calculates kills and morale loss.
Targeting - Normally units engage enemies directly in front of them but they can attack any unit within their flanking range if doing so will result in greater damage to the enemies combat ability. Artillery can fire from the back row but only do 50% of their potential damage from there.
Morale - Units take morale damage for every day in combat (including reserves), with greater loss for receiving actual damage. Once it hits zero, the army attempts to retreat. At this point, two more fire and shock phases are played out and if the retreating army has zero morale and is outnumbered 2:1 before those are completed it is destroyed in a stack wipe. As this is such a critical stat, efforts should be made to increase morale where possible.
Discipline - An often forgotten stat with only limited small modifiers it is nonetheless important. Damage dealt is multiplied by your discipline, and damage taken can go up or down depending on the percentage.
Supply Limit All provinces have a supply limit based on the terrain, development, and various modifiers. As an example, an army leader with 2 pips in manoeuvre will increase this limit by 2 for their army. As a guideline you will get the biggest positive modifiers on coastal zones, then as the owner of a province, then with allied/access/controlled provinces, though the biggest bonuses come from military technology levels.
Attrition - If the army size of all non-hostile armies is above the province Supply Limit at the beginning of the month, they will take attrition damage. Attrition damage is capped at 5% maximum, but scales depending on how much you are above the limit and what the limit is. For a simple example, in a 40 supply province, you would suffer 0.25% attrition (10 divided by the supply) for each point you exceed the limit by, so going over by 4 units would lose you 1% per month. A modifier that reduce attrition by 25% would reduce this to 0.7% (multiplicatively maths, just go with it), while also capping the attrition at around 4%. Sieging a province adds 1% attrition no matter what, which would increase our example to 2% - it all adds up.
Reinforcements - This is how fast you can pull men from your manpower pool and have them join your weakened armies. There is a massive monthly ducat cost to reinforcing armies so speed modifiers can save you a lot of money. This cost only applies if your army is below max strength, so when dealing with attrition, if you can send enough men to cover the losses there is no additional cost. That said, attrition can rapidly deplete your manpower through stealthy reinforcement, so keep an eye on it closely.
Attack and Defence - The defender almost always gets an advantage in battle, so luring the enemy to you, or tackling them while they are sieging your provinces (which counts as defence) is the best bet.
Terrain - Terrain penalties can be harsh on attacking forces, so where possible avoid mountains, hills, and woodlands if you really must engage. Mountains give -2 to the attackers dice roll, essentially cancelling out early game generals. Hills and woodlands are only -1 but these modifiers effect both combat phases so best to avoid.
Crossings/Landings - Same as the terrain really. Straits and naval landings give -2, and rivers -1. In the case of multiple attacks from different directions, the worst modifier is used for the whole attacking force. If the attacking leader has higher manoeuvre than the defender, this penalty is negated, so high manoeuvre pips are excellent for attacking armies.
Professionalism and Drilling - Both provide insane bonuses to your army strength at 100%. Professionalism can net you 20% to siege ability and 10% to combat damage in both phases. Various milestones also unlock abilities like Supply Depot that can massively increase your supply limit in an army location and will let you play with bigger stacks. Drilling has even better numbers but no milestones. It's also tied directly to a single army but having another 10% to damage dealt, and a staggering 25% reduction to damage received makes drilling a no-brainer whenever you have the option.
Army Tradition - Grants morale and manpower bonuses but you'll often get hit with events and natural decay to counter it getting really big. Great if you can be bothered with it and the morale boost really helps winning those otherwise equal battles.
Other Modifiers - There are too many modifiers to go into here, but stacking them up pretty much works the same regardless of your choices. Pick what suits your playstyle but if you're looking for the perfect army, focus on the military ideas and don't forget to synergise with policies for some too big to believe bonuses.
Infantry - The grunts of any army composition, they deal the damage and take the hits, suffering the highest casualty rates. They are the cheapest unit and you always need them. As mentioned above, once artillery comes into play, pick morale and defensive pips.
Cavalry - The flankers that sit on the ends of the front line. They will fill spaces in the back but only after artillery and infantry have run out. Quite expensive for what they are and only really useful if you have a greater combat width than your opponent or you're tackling armies that don't fill out their own front lines. They become increasingly redundant as the game goes on but you can't really go wrong with a handful in each stack. If you're facing smaller armies where you can make good use of flanking bonuses, offensive pips are useful here.
Artillery - The big guns. They are the only unit that can fire from the back, although they do so with a 50% damage reduction. Either way, being on the back line means they won't take damage until they are forced to the front by loss of infantry. Very expensive to buy and maintain being three times the cost of infantry. At later levels, there's no choice for artillery but what you're given, so pips don't matter much.
The optimal composition is talked about a lot and in many ways it is set in stone, and while we'll go over it and why it's so good, we'll also look at why it isn't always what you want.
For that perfect storm of battle prowess, you want to be aiming to fill the entire back line with artillery and the majority of the front with infantry, leaving perhaps two spots free for cavalry. There are also variations based on what stage of the game you're in, but for the majority of the game - and in the common vernacular - you're looking at a 10/2/12 ratio, scaling to your combat width and replacing excess cavalry with more infantry. This ratio will give the army the single greatest opportunity to win an army of that size would get, everything being equal.
The problem with this comes from many of the things we talked about earlier, firstly the damage suffered by infantry. If you lose 4 units from a 12 length line, 2 of the cannons move forward to even out the formation, exposing them to fire.
You can counter this somewhat by adding more infantry to the composition, but this usually involves hitting supply issues. Let's take an reasonable mid-game enemy province giving you a supply limit of around 30. At that point in the game you'll likely have military technology giving you a line width of 30 as well. Even following the basic ratio, you're going to want to fight with a total of 60 regiments (not counting reserves). Given a general with 3 manoeuvre pips, you're still 27 over the limit. Using the maths we talked about earlier (10/30=0.33)x27=9 we get the maximum attrition (5% or 4% with defensive ideas). 5% of 60,000 men is 3,000 casualties every single month. Over a 12 month campaign, that's 36,000 dead from attrition alone, in each army.
Split stacks I hear you say? Well yes, we can do that, but if you're splitting your stacks, you're also halving the artillery that can be deployed. If you're wanting to maintain a full front line or have infantry reserves for your merged army you'll probably end up with a full 30 infantry/cavalry and 15 artillery in each smaller stack. You're still looking at an army stack of around 45, while sacrificing artillery support if you fight alone.
But would you fight alone? Probably yes, it happens... but isn't the point of splitting stacks to avoid attrition until you can begin a battle - when attrition is placed on hold - and pile them back into one? Yes, but you'll also be using smaller stacks to siege and chase down smaller forces, and sometimes they can get cornered.
From this we can already see the magic ratio is broken. We know the optimal battle composition, but we've learnt that it fails in terms of war composition. Even split stacks will often face issues with supply. So the test is figuring out what you can get away with at the time. We take the average supply you'll be facing in a given conflict (how big your armies should be to avoid taking the heaviest penalties), how many stacks you intend to merge for a fight, and how they will cope fighting alone.
You usually want to max your front line, so whatever that number is make sure you have enough infantry and cavalry to fill it out. In our example that's 30. If we had split the stack as before it would be hitting 45 with artillery and taking pretty hefty attrition in a 30 supply province (pretty much the full 5%). If we lower the artillery to just 10 though, it drops attrition to 3.3%. With these army sizes that lowers your 12 month loss from 27,000 men, to only 15,840. Okay, so that's still a lot but it's a lot better than before, and all for dropping only 5 units. 10 artillery is also the prime number for mid-game siege bonuses (although the bonus is based on per thousand men not the number of units, so any casualties that aren't reinforced will reduce your siege capability).
You could further reduce this attrition by decreasing your infantry numbers below the combat width, but that becomes increasingly dangerous if that army is ever confronted alone where it will face flanking penalties. If you do go this method, remember as well that though you may start out with enough reserves once the armies join together, over the course of a war you may find reinforcement can't keep up with your losses which means it pays to have extra infantry around anyway.
Even so, we're now talking about having only 10 artillery included, the perfect sieging stack for mid-game or lower level forts later on, and reasonably sized to avoid the worst of attrition. With only a third of our combat width in artillery though, we'll want to be merging 3 armies for any battle, which is definitely doable, if a little awkward when dealing with enemy zones of control. Does this mean we now have too many infantry? This depends on how much you intend to use them solo, and remember, the larger the stack, the lower the chance of getting wiped.
Does this make it the ideal composition? Another loaded question as it all varies on your circumstances, as having lots of manpower could offset your attrition losses but lets look at it from one final angle - the economics of war.
Economics of War
As with supply, money is limited. Ducats are needed to finance everything from buildings to your military campaigns. Every ducat a month you spend on a unit is one less you have to invest during peacetime. You can slide the Army Maintenance way down to save a bundle when you don't need them, but you should always be drilling your forces to full, so you can't really cut maintenance if you want to maintain an optimal army.
Artillery costs three times more than infantry to buy and maintain, so if you're struggling for money but have some spare force limit, padding your armies with infantry makes even more sense. As infantry also take the most damage in wars, you can use extras to artificially increase your manpower pool by consolidating them directly into wounded units rather than drawing from reinforcements. With this you can get great value for money.
To wrap it up, we've seen there is a perfect composition and a best unit to pick from each type to give you the greatest chance of winning a battle, but in practice tying yourself to that will often cause more trouble than it's worth. Best bet is to ensure that your merged armies will have enough artillery to fill the back line and whatever number of splits you make are able to minimise their attrition while still being able to fight alone if they have to.
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