Tank
Most modern tanks most often have four crew members, or three if an auto-loader is installed. These are the:
Historically, crews have varied from just two members to a dozen. For example, pre-World War II French tanks were noted for having a two-man crew, in which the overworked commander had to load and fire the gun in addition to commanding the tank. First World War tanks were developed with immature technologies; in addition to the crew needed to man the multiple guns and machine guns, up to four crewmen were needed to drive the tank: the driver, acting as the vehicle commander and manning the brakes, drove via orders to his gears-men; a co-driver operated the gearbox and throttle; and two gears-men, one on each track, steered by setting one side or the other to idle, allowing the track on the other side to slew the tank to one side.
Tank destroyer battalion (United States)
The first nine units were deployed during the Louisiana Maneuvers of 1941, equipped with towed 37 mm anti-tank guns (the largest gun that could be towed by a jeep) and 75 mm guns mounted on half-tracks (the M3 Gun Motor Carriage), and again in the Carolinas maneuvers that November. Their employment was judged a success—though not without some disgruntled voices in the armored branch arguing that the umpires had rigged the results—and on 27 November, Marshall ordered the establishment of a tank destroyer Tactical Firing Center at Fort Hood, Texas, under Bruce, and the activation of 53 new anti-tank battalions under the command of General Headquarters. The term "tank destroyer" was used from this point on, as it was seen as a more psychologically powerful term. On 3 December, all existing anti-tank battalions were reassigned to General Headquarters and converted to tank destroyer battalions.
The new tank destroyer doctrine was formally stated in Field Manual 18-5, "Tactical Employment, tank destroyer Unit", in June 1942. It laid down a focused doctrine—"There is but one objective of tank destroyer units… the destruction of hostile tanks"—and repeatedly emphasized an offensive, vigorous spirit. Tank destroyer units were expected to be employed as complete battalions, held in reserve and committed at critical points, rather than parceled out as small defensive strongpoints. The emphasis was heavy on mobility, and the ability to outmaneuver the attacking armor; this would feed into vehicle design, and cause the speed of a vehicle to be prioritized over survivability.