An annexation ally is an AI ally that a human player uses as a proxy to force-annex enemy single-province countries. The main reason for using a proxy is the high cost in badboy of annexation. However, annexation also causes a significant drop in diplomatic relations with other countries, which can be another reason to avoid it.
Any time you are faced with a lot of single-province minors which you'd like to absorb, but you are concerned with badboy, then you should consider using an annexation ally to do your dirty work.
Cleaning up Your Core
When the provinces you are after are your core provinces, it is often better to use a non-vassal annexation ally. After you've helped it annex all single-province countries that are in your core, then you eject it from your alliance, declare war, and take all of your core provinces from it in a single war.
Note that it is often possible to use an annexation ally even against non-single-province countries in your core. Here the scenario runs as follows:
- You DoW. Eliminate the enemy's army, and cover all provinces.
- You control all sieges, ending up with 100% warscore. It is often best to leave the siege of the enemy capital to last, and wait for your ally to get an army there before completing it.
- You make a separate peace, taking all provinces from the enemy (except its capital), and all its money.
- Your ally is still at war with the enemy, and can now hopefully complete the siege and annex.
Growing Outside of Your Core
When the provinces you want are not in your core, then you'll typically want to use a vassal to do the annexation. On the one hand, it's often harder to get a vassal to annex (see below). On the other, for you to get the provinces, you are going to have to diplo-annex, which requires the annexation ally be a vassal eventually. Having the annexation ally already as a vassal gets you most of the way there.
Setting up Annexation
For a non-vassal annexation ally, getting it to annex is easy. AIs always annex when they can, so your ally will annex the enemy as soon as it completes the siege. Start your siege normally, but when the defenders' siege number gets to -3 or worse, arrest the siege, waiting for your ally to get around to helping. When it does, immediately hand off the siege and let the ally take control of the province.
When your ally is your vassal, setting up an annexation can be more difficult. The problem is a rule introduced in EU2 1.09: a vassal cannot make a separate peace when in the same war as its suzerain. Annexation counts as a separate peace in this regard, so the problem is that your ally won't annex.
To get around the problem, you need to split the war, so that your vassal is in a different war than you are. This means you must make a separate peace to split the war.
One way to do this is to first siege the enemy yourself, keeping control of the siege. Once you have taken control of the city, you can make a separate peace, taking all of the enemy's money. Then you hope that your ally can complete the siege on his own. It is usually best to arrest the siege in this case, and wait for your ally to decide to help. When he does, and you complete the siege, then at least his army will be in the right place to resume it.
A second way to handle things is to let your ally control the siege and take control, and then make a separate peace with the enemy. In this case, your warscore will not be 100%: your ally controls the enemy's province, not you. Often your warscore is 0%. Thus it can sometimes take a few offers to get a peace (usually, a white peace). However, the enemy AI does not know that making peace with you is certain death for it, and eventually will accept peace with you, leading to its annexation.
Another way to handle splitting the war, when there are are least two countries in the enemy coalition, is to make your separate peace with the other one. In this case, you should let your ally siege down the country you want annexed, and then make the separate peace with the other to split the war.