The introduction of Armor meant that a means now existed to break through enemy positions and to move aggressively forward on the battlefield. What was needed, however, was a force which could rapidly deploy into enemy territory and then defend the ground which had be won. Mechanized Infantry, first used during World War II, so proved its worth that by the end of that war all of the major combatants had developed large formations of this type. Infantry were mounted in armored vehicles of their own, such as half-tracks or APCs (Armored Personnel Carriers), to carry them quickly and safely along with the rapidly advancing Armor.