Strikes can be declared on enemy islands every 8 hours by Alliance Generals and Officers only. Your Strike targets need to be directly next to an island held by your own alliance. Strikes can only be declared if the War itself still runs for longer than the Strike would take.
By successfully defeating an enemy player of the attacked Alliance during a Strike, the amount of Victory Points displayed next to his name will be added to your Alliance’s score for this specific strike.
At the end of the strike, the winning alliance is determined by who earned the most overall Victory Points. If the attackers win, they take over the enemy island. If the defenders win, they get to keep it.