Military access with enemies
Having military access with your enemies can make wars quite a bit easier. But there's also several other exploits of it worth mentioning.
Exploits and Uses of MA
Military access gets you two things: more supply, and being allowed there in peacetime. Here's a list of exploits that military access makes possible:
- In war, lowering attrition because of the increased supply. Note that MA-enhanced supply in a port province is doubled normally by a blockade, making it possible for you as an enemy to actually have more supply in a province than the owner has!
- In war, capturing unfortified ports with a fleet.
- In peace, exploration of the other country and adjacent owned provinces. See how to get maps. It is also rarely possible to capture the other country's capital (from rebels), and thereby gain all of its maps.
- In "peace", exploitation of natives to burn trading posts and destroy colonies, opening up the land for your own colonization.
Getting MA (almost) Everywhere
How do you get military access with an enemy? Well, if diplomatic relations are already bad, then you probably don't -- at least, not via diplomacy (see below). But often at the start of the game you'll have many more diplomats than you have use for, and you haven't yet ruined your relations with most other countries via the nasty sorts of things you intend to do. Very well: use all those early diplomats to get military access. Get it with everyone, except countries which you already have plans to attack. Generally, you can get military access from most countries that are your same size with just a request, if relations are good (100+). Larger or more paranoid countries may require 150+ relations. So long as relations are positive, you can sometimes (though rarely -- keep asking) get MA with more powerful countries. If you are powerful, you can get MA with most small countries with one request down to about relations of -25 or so.
You especially want to get MA with the major powers, when they are still small. I find that with a Catholic country I can usually get MA from Portugal, Castile (before it becomes Spain), and England. France is a maybe, and Austria is very difficult due to its diplomatic monarchs. But other medium powers (Lithuania, Poland, Denmark, etc.) should be easy.
If you can't get MA with a country diplomatically, then you can get it militarily. Obviously this requires one successful war, and you may not always want it. But you should always have it in mind for any peace. Taking MA does not cost badboy.
Why Not Everywhere? The MA Bug
With many benefits, and a small downside, getting military access with every other country seems like a good idea. But, there's a catch: there is a known bug in EU2, still present in 1.09, that if you have MA with more than 50 countries, whenever you select the diplomacy screen on your own country, the game crashes. So: you must limit yourself a little bit. If you do get into this state by accident in a game, the thing to do is edit a savefile of your game to remove some of the MA agreements you've gotten. In the savefile, you'll find a "diplomacy" section for your country, with "access = yes" on many of the lines -- just remove some of the "access = yes" statements and the game should work normally.
Getting in Wars with MA
How do you get into wars if you've got military access everywhere? Well, the typical method is get in a war with a desired target country indirectly. Instead of DoWing the target country, you select one of their allies. Cancel MA with the ally if you have it, and DoW them. (Ideally, you should have casus belli against the ally.) The desired target will honor its ally's alliance call, except if you have better relations with it than it does with the ally (and even then, it usually will honor the call). If there's any doubt, then you can always send an insult or two to the target to get relations down.