[st.patrik]

If you are already at war to my mind the best thing to do (in SP) is to drive straight to their capital and siege it. This is basically the theory of blitzkrieg. Often they will bring everything back to try to liberate their capital, thus meaning your provinces aren't getting rampaged. The other thing with the AI is that often it's pretty predictable where they're going to attack - pretty much if you leave a hole in your defenses they'll attack there. This means that you can bring about a confrontation by offering up an inviting 'hole' and then moving all your forces into it the turn they attack.

In MP of course it's different. People will still tend to rush to protect their capital, but might be smart enough to keep an aggressive edge too. I guess my main advice is that if you're at war you need to stop thinking about preserving your borders and start thinking about winning the war - which are very different things. This also depends on whether they have an objective - like taking your capital, or if they are just raiding your borders to cause trouble, or whatever. In general if I think I can take their main army I try to bring about a large battle, and at the same time have a couple of guerrilla forces picking away at their provinces/supply lines to ensure that I keep something of a gold income. If I think I can't take their army I just do the latter, while building a force which I think might be able to face them.

hope that's helpful