BT-7

The BT line began with J. Walter Christie, an American engineer whose tank designs the US Army persistently declined to buy. His suspension — large road wheels on long independent coil-sprung arms — allowed high speed over rough ground, and his vehicles were convertible: the tracks could be removed and drive transferred to the rearmost road wheels, turning the tank into a very fast wheeled vehicle.

The Soviet Union bought two of his machines in 1930 and developed them into the BT series — bystrokhodny tank, fast tank. The BT-7 of 1935 was the mature version, built at the Kharkov Locomotive Factory in roughly 5,000 examples out of a BT family of some 8,000.

The convertible feature sounds like a curiosity and had a serious purpose. Tank tracks in the early 1930s were unreliable and short-lived, wearing out in a few hundred kilometres, and a formation that had to drive a long way to reach a battle could arrive with half its vehicles broken. On wheels the BT could cover roads at up to about 72 km/h without touching its tracks at all.

In practice a crew needed around thirty minutes to make the change, and the wheeled mode was useless off-road, so the feature was progressively judged not worth its weight and complexity — it disappeared from the design lineage. But it explains the shape of the tank: the enormous road wheels that give the BT and, after it, the T-34 their unmistakable look are there because the wheels originally had to carry the vehicle by themselves.

The BT was the instrument of a specific theory. Soviet deep operations doctrine, developed by Tukhachevsky and others, envisaged breaking the enemy front and then driving mechanised forces far into the rear to collapse the whole system rather than to win a local battle. That requires tanks that can go a long way, quickly.

The theory was sound enough that the Wehrmacht arrived at something similar, and the Red Army spent the late 1930s dismantling it: Tukhachevsky was shot in 1937, the large mechanised corps were broken up, and the doctrine fell out of favour just before the war that would vindicate it. The tanks remained. The organisation and the officers who knew what to do with them did not.

The BT’s best-known action came in the summer of 1939 at Khalkhin Gol, on the Manchurian frontier, where Zhukov used massed armour to encircle and destroy a Japanese force. BT-5s and BT-7s made the deep flanking drives that closed the trap — the doctrine working exactly as designed, in the one place it was allowed to.

The cost showed the flaw. Japanese infantry attacked the tanks at close quarters with petrol bombs, and the BT’s aviation-derived petrol engine and thin armour made it burn readily; some Soviet accounts of the battle dwell on how quickly a hit tank became an inferno. The lesson pointed directly at diesel, and the Soviet answer — the V-2 diesel — was already being developed for the tank that would replace it.

The BT-7’s importance is what grew out of it. The design bureau at Kharkov, working to replace the BT, produced the A-20 and A-32 prototypes — still Christie-suspended, now with sloped armour and a diesel engine — and from those came the T-34. The lineage is direct and physical: the T-34 rides on the same suspension principle, and its large road wheels are the BT’s road wheels grown up.

A late variant makes the point in one vehicle. The BT-7M fitted the new V-2 diesel to the old airframe, and it was the end of the line for the BT and the beginning of the engine that would power Soviet tanks for the rest of the war and long after it. The BT-7 is where nearly everything that made the T-34 was tried out.

Its own war ended as the T-26’s did. Thousands of BTs were in service in June 1941, in the mechanised corps that had been hastily reconstituted, and they were destroyed in the same weeks and largely in the same way: outgunned where they fought, and abandoned in far greater numbers for want of fuel, spares, recovery vehicles and orders. Fast tanks are of no use when there is nowhere coherent to go quickly.

The verdict has to hold two things at once. As a fighting vehicle in 1941 the BT-7 was obsolete — thin, petrol-fuelled, under-armoured — and it was expended without achieving much. As a design it is one of the most consequential of the interwar years: an American suspension the American army rejected, developed by the Soviet Union into the running gear of the most influential tank of the war.

• Steven J. Zaloga & James Grandsen, Soviet Tanks and Combat Vehicles of World War Two (Arms & Armour Press).
• Steven J. Zaloga, BT Fast Tank: The Red Army’s Cavalry Tank 1931–45 (Osprey, New Vanguard).
• David M. Glantz, Stumbling Colossus: The Red Army on the Eve of World War (University Press of Kansas).