Base wars is simply a 4-team minigame (Can have 2 or 3, but best with 4 teams :D) where you go from your base, using blocks and weapons from your base and other areas to eliminate the other teams. To stop their re-spawning ability, simply break their crafting table at their base. Other than that, have fun.

On a side-note, the game is run by command blocks and around 15 minutes in, the map goes into "Phase 2" where chests are restocked with better items and the fighting gets more intense.

Best with 4+ Players (Odd numbers can get unfair, but either way it'll work. Have fun!)