of forcing a breakthrough into enemy rear areas. Separate GHQ tank battalions would support infantry in destroying fixed enemy defenses, and armored divisions would then exploit the breakthrough to rush into the enemy's vulnerable rear areas. U.S. tanks were expected to fight any hostile tanks they encountered in their attack, but the mission of destroying massed enemy armored thrusts was assigned to a new branch, the Tank Destroyer Force developed in 1941. Tank destroyer units were meant to counter German blitzkrieg tactics. Rather than rely on a "thin cordon" of anti-tank guns which would be defeated by a concentrated attack, the tank destroyer units were to be held as a reserve behind the battleline (at the corps or army level), and were to move quickly to the site of any massed enemy tank breakthrough They would maneuver aggressively and using ambush tactics to destroy enemy tanks; charging or chasing enemy tanks was explicitly prohibited. This led to a requirement for very fast, well-armed vehicles. Though equipped with turrets (unlike most self-propelled anti-tank guns of the day), the typical American design was more heavily gunned, but more lightly armored, and thus more maneuverable, than a contemporary tank. The idea was to use speed and agility as a defense, rather than thick armor, to bring a powerful self-propelled gun into action against enemy tanks. Or more precisely, to use speed to deploy ahead of the attacking enemy, take up camouflaged and protected firing positions on their flanks if possible, and then open fire. If unable to destroy the enemy force or to force them to retreat, then mobility would be used to avoid enemy fire until the TDs could withdraw, preferably to move up and deploy for another ambush. Direct combat in the open against tanks was to be avoided whenever possible. Development After the formation of the Tank Destroyer Tactical and Firing Center at Camp Hood, Texas in November 1941, the Army began testing to standardize on a configuration for the new tank destroyer battalions. The Tank Destroyer board began to examine several hundred Ordnance Department prototype proposals for a tank destroyer mounting a 3-inch gun, initially focusing the most interest on two: The T1, developed beginning in 1940 and standardized as the "M5 3-inch Gun Motor Carriage" in January 1941. It was a modified Cleveland Tractor Company airfield towing tractor mounting a 3-inch gun T9 in a limited-traverse mounting T24,