Battle is a fundamental part of any successful Europa Universalis II game. And newbies, especially, often struggle with it. This article is designed to help all players improve their warfighting skills by learning how to win battles.
What is "Winning"? Edit
One important thing to understand first is what it means to "win" a battle. In a technical sense (that of warscore), you "win" an EU2 battle if you destroy the enemy or if you force the enemy to retreat. But this is narrow conception of winning, because it does not account for losses. Losses for the AI are cheap, and easily replaced. For the player, they often aren't.
Rather, within this article, to "win" a battle means you win in the warscore sense, and in addition, you do it without taking unacceptable casualties.
General Tactics Edit
First, you need to understand the combat system. There is randomness in combat, but it happens only every 5 days. So, the first roll that happens has its effects imposed over 5 days, but you can see what it is (more or less) after the first day of battle. It is highly important to observe the first day of battle and flee that battle if the first result is at all against you.
The Probe Edit
Here is the most important tactic in the game; it's called probing. Try to fight only one major battle at a time. When your forces enter battle, you get a message; set those messages to automatically pause. Go to the battle, and examine your force and the enemy force on the battle screen. Note the attacker's DRM, and the size of both forces. Unpause and let the game run for exactly one day and no more, then pause again. Compare the effect of the day's result on both sides. If the effects are all against you (which is likely unless you've got a good DRM or technology edge), then retreat!
- Observe the first day of battle and flee that battle if the first result is at all against you!
If you do stick in battle, pause every 4 days and single-step a day, while you watch the results. This is when the reroll happens, for next phase. Again, you want to flee if the thing turns against you.
Using Reinforcements to Keep Morale High Edit
Combat in EU2 is very sensitive to the morale of the two sides. It is not known exactly how this works, but it appears that low-morale forces get negative DRMs; it may also be that having a higher relative morale gets a positive DRM. In any case, it is far easier to win a battle if you can keep your morale high.
Combat is EU2 works counter-intuitively in that the "morale damage" an army inflicts is completely unrelated to its size. 1 man or 1000, or 100000 men: all inflict exactly the same morale damage. (This is why tiny AI forces can sometimes beat your huge armies.)
Thus, it is a good tactic to send reinforcements to a battle keep up morale. When reinforcements join a battle, their side's morale goes up as the weighted average of the morale of the existing units, and those entering. Thus, to keep morale high you need to feed in increasingly large forces. My rule of thumb on this is to start with very small forces, 500 or 1000 men, then send roughly double the force each 5-10 days. However, if the battle must be won (that is, you don't want to probe), then you should send in initial forces of no less than perhaps 1/10 to 1/20 of the size of the enemy army, because smaller forces can often be killed completely in a phase or two of combat.
For example, you have a stack of 5000/5000 infantry/cavalry. You want to attack an enemy army. You might just send all 10000 men together and hope for the best. (At least probe with them.) But it would be better to do as follows:
day 1 of a month: send 500 infantry day 5: send 500/1000 infantry/cavalry day 10: send 3000 infantry day 15: send remaining 1000/4000 infantry/cavalry
When you have higher technology than your enemy, you usually have higher morale to begin a battle, and this tactic becomes less important. And doubly so if you have a fire phase and the enemy does not, since you get to knock down his morale without retribution. If you do use reinforcements, you can send fewer groups, usually just one large reinforcement group.
Conversely, if you are fighting with lower technology and morale, using reinforcements is necessary to win more than a tiny fraction of battles.
Fight with Advantageous Die Roll Modifiers Edit
Know how to get good die roll modifiers, and only fight with them. Domestic policy settings, leaders, the cavalry bonus for 2x the cavalry in plains/desert, and defense across rivers are the main ones to look for. Only engage the enemy with bonuses -- if you don't have them, maybe probe for one day of battle (to see if you get lucky) and flee, or even just flee outright. Try to get Offensive 9 (in the early game; until about 1500 or so), and Quality 9 (after land tech 9), as soon as possible.
Don't Be Afraid to Run Away Edit
You can make up for the warscore losses from all the battles you lose by (a) killing lots of small armies immediately upon their recruitment, when their morale is low, and winning is easy, or (b) getting 100% warscore. If you will get 100% warscore anyway, it does not matter how many battles you flee. Also, the maximum loss of warscore to one enemy due to battles is 99%. If you flee 1000 battles, it would not matter; if you could win 99 battles against new recruits, your warscore would be even.
Remember, as a human you are constrained by the money cost of armies; at the higher diffulty levels, the AI isn't. Therefore, you must carefully husband your precious armies. Running is a key to that. Making up 1% in warscore is easy. 10% is possible, given an army to do it with. Making a new 10000 man army is often not possible, and even if possible, very costly.
Use Infantry to Absorb Losses Edit
Cavalry is very powerful in the early game, and in terms of power per ducat they are a much better bargain than infantry in the early game. You should buy mostly cavalry until you get to land tech 9. Nonetheless, infantry are always cheaper per man than cavalry, and according to the rules for attrition and for combat, when multiple types are in an army together and casualties are inflicted, most of the losses will be allocated to the infantry (5/6, to be specific).
Note that simply having infantry and cavalry together in the same province is not enough to get the "soak off" effect: they must be in combined armies.
Thus, at least in provinces which you control, you should generally not fight with pure cavalry armies. Instead, use a few thousand infantry with each large cavalry army (often 10000 to 20000 men) to absorb losses. After each battle, you'll usually need to replenish the infantry, and sometimes the cavalry.
In provinces you do not control, if you move in an army combining cavalry with infantry, the whole army will be slowed until either it can get into controlled terrain to recombine, or the infantry all get killed by attrition or combat. Thus, you will have to use your judgment. Pure cavalry has the advantage of speed, which is often desirable. But if you can fight some or most battles with infantry to soak up losses, so much the better.
Specific Tactics Edit
Here are some tactics for use in limited situations.
Handing Higher-tech Infantry with Primitive Forces Edit
A "primitive" force for this purpose is any force with zero firepower. This includes everyone at the start of the game, but later on is just pagans and natives, and the occasional really isolated country. A primitive force always has a CRT disadvantage against a force with firepower, but that is not the real cause of its problems.
The thing about firepower is that the morale effect of combat (unlike the killing effect of combat) affects the entire enemy force present, without regard to its size or the size of the firing force. Whereas, the killing effect of combat is proportionate to the size of the firing force. Thus, a higher-tech army with firepower can often rout a primitive enemy force no matter how large it is. And no matter how small the higher-tech army is -- a single man infantry army can sometimes rout 30000 Aztecs.
So, how can you deal with firepower as the primitive force? What you want to do is soak up the "morale damage" from first fire phase with a token force. Since you can't fight back in the fire phase anyway, this force is there only to absorb the morale damage and to keep the battle going long enough to get to a shock phase. Thus it need only be a few thousand men, depending on how large the enemy is. After the fire phase ends, which is four days after the battle starts, you arrange for your main army (ideally, mostly cavalry) to arrive. (Obviously this requires micromanaged moves leaving on specific days.) Because the main force wasn't there earlier, its morale is still full, and the overall morale of your force is now almost at maximum. So your men will fight for the shock phase, and a decent roll means you'll eliminate the enemy.
Fighting Primitives with Firepower Edit
The converse of the situation above, is when you're the force with the firepower and you're attacking a primitive force. In this case, you want to maximize the impact of your firepower, and you'll want to fight a "shock-probe". A shock-probe is like a normal probe, except that you do not even need to pause when the battle starts -- you know that you can't be touched in the fire phase. Rather, you wait until the first shock phase (the 5th day of the battle) to pause and watch carefully. If the enemy gets unacceptably good die-roll, you flee.
You do not need any substantial amount of firepower to get its morale effect. Even a single infantryman with a stack of cavalry can be very helpful in gunning down a primitive stack's morale.
Note that with a sufficient advantage against primitives, it becomes reasonably likely to rout them in the first fire phase. This makes it reasonable to fight battles solely with small infantry armies (250-500 men), or even tiny ones (1 man).