During World War II, ground-attack aircraft emerged as a significant threat to mechanized units on the move. Conventional towed anti-aircraft artillery (AAA) was an inadequate response under such conditions owing to the time needed to bring anti-aircraft machine guns into action. This experience made it clear that an anti-aircraft tracked vehicle, armed with small-bore autocannons or heavy machine guns, was needed. Vehicles such as the German Wirbelwind and specialized variants of the US M3 Half-track had been used to good effect in the final battles of World War II, both by the US and nations which had received the M3 through Lend-Lease.