attack

on the attack

forcefully criticizing or energetically competing against someone. Flynn went on the attack against his rivals, finally bringing some life into a very dull campaign.

Etymology: based on the literal meaning of on the attack (= using force against an enemy)

under attack

being acted against physically or with words. She said cuts in spending put education in America under attack.

under fire

Etymology: based on the literal meaning of under attack (= having physical force used against you)