The introduction of armored tanks meant that armies could quickly break through enemy defensive positions and move aggressively forward on the battlefield. Such a quick advance, however, left the ground won in the advance largely undefended. Mechanized infantry, which was first used during World War II, was designed to move in quickly and defend the conquered area behind the tanks. Infantry were mounted in armored vehicles, such as half-tracks and Armored Personnel Carriers (APCs), to carry them quickly and safely along with the rapidly-advancing armored divisions.