Nice, simple WW I simulation
There are scenarios and the "Conquest" game – all of WW I in Europe. In all of them, the player commands a side, and the player + possibly allies must conquer all of the enemies' land squares. This means if you are the Germans in the Jutland scenario, you must not only defeat the British fleet, but also conquer the part of England in play as well. Sealion comes to WW I.
The scenarios are divided into the Empire and campaign games. For the empire one, you pick a country (staring with England) and play a series of scenarios with it in order. The scenarios aren't particularly historical – First you have to invade France across the channel, then next game go over to North America and help the Canadians defeat the USA, and then go back across to France later to help fight the Germans. The campaign games seem to focus on the area rather than sides, but I don't know because there seems to be some kind of bug where it keeps putting me back in the first battle after I've completed it.
The game doesn't always tell you who is on what side, who is your friends, who is your enemies, and who are neutrals. You have to watch as battles go on around the map. Any one that attacks one of your units or invades your territory is an enemy. Anyone that attacks a known enemy is an ally. If a nation is on your border and doesn't attack you, it's either a neutral or friendly.
In practice, this doesn't matter much because your allies are very tolerant of you attacking them. I think this is because frequently friendlies are blocking your way to enemy territory and you would be stuck if you couldn't attack them.
The AI does passibly well in strategy, the computer players usually only attack something nearby, and gradually shuffle over wherever. It does quite well tactically. It understands how to best attack with each kind of unit. Machine guns do a second round of fire (if they survive) on infantry and cavalry, artillery only gets shot back at by artillery or units in forts, etc.
The biggest fault I've seen in this thing so far is the lack of documentation. What tutorial there is is very general, and no formal manual. You have to learn by trial and error. The game is not all that complicated, but learning the nuances can be a pain, such as what the little circles on the map mean, or what it means when units turn red or blue.
I've played about 15 games so far, at least 30 hours of play, and it's generally been pretty good. I like the WW I conquest scenarios the best, been trying out the various sides. There is a free version of this game, I encourage you to try it out.