Annihilation is a game mode used commonly in Basic Matches and Skirmishes due to its popularity over Victory Point mode which typically results in an offense/defense situation, and thus is deemed more fair and faster moving. In Annihilation mode,your basic goal is to defeat all Enemy emplacements, hence the name, including Non-base structures such as MG Emplacements,Triage Centers, and bunkers. A common tactic is to eliminate unit-spawning buildings before destroying upkeep-buildings and then the HQ.
Arguably,the best team for Annihilation is the British,as they can move their HQ and other unit-spawning vehicles if in trouble.
Note that you do NOT have to destroy Observation Posts to win.