The premise behind Jagged Alliance is that you control a team of mercs. You hire them, guide them through combat, and pay them. They have different strengths and abilities. The game is laid out in sectors, and your objective is to eliminate the enemies from each sector. When a sector is clear, you can move on, and in JA 1, you can hire guards to keep it from being re-invaded (unless all of your guards get killed, which isn't uncommon). What happens most of the time is you'll be happily killing your way in one direction, and the enemy will attack sectors you've already completed from a different direction, and retake them. Then you have to go back and take that sector again (pain in the friggin butt).
Also, you run into stuff like merc who don't get along (Ivan and Reuben . . heh heh). And, after a while, your equipment begins to break down. Not fun. The things you do affect how your mercs will behave, and how the enemy will behave, and how your employers will pay you. It's all very interesting, and I thoroughly enjoyed JA 1. Now if only I could get JA 2 to run on my machine, I'd be much happier.