Defeat the enemy by destroying their stuff.

You lost all your planes and the island was captured by the enemy, but at least you tried.

You sank a loaded transport.

You sank a battleship. I hope you're proud of yourself.

You sank an enemy carrier.

You sank a destroyer. Well done, although it's not really that difficult.

You hit an enemy tank or something with your gun.

You downed a cruise missile somehow.

You successfully landed your plane (which sinks) in the sea.