text: In June 1941, the European Axis powers launched Operation Barbarossa, invading the USSR. The following month, Britain and the Soviet Union formed an alliance, the Anglo-Soviet Agreement. Britain was quick to provide materiel aid to the USSR beginning in August - including tanks and aircraft - in order to try to keep her new ally in the war against the Axis powers. One major conduit for supplies was through Iran. The two nations began a joint occupation of Iran in late August, to neutralize German influence. The Soviet Union joined the Second Inter-Allied Conference in London in September. The USSR thereafter became one of the "Big Three" Allies of World War II along with Britain and, from December, the United States, fighting against the Axis Powers. The American Lend-Lease program was signed into law in March 1941. It provided Britain and the Soviet Union with limited war materiel beginning in October that year. The programme began to increase in scale during 1943. The British Commonwealth and, to a lesser extent, the Soviet Union reciprocated with a smaller Reverse Lend-Lease program.
  Convoy organisation
  After the first convoy, code-named  Operation Dervish in August 1941, the Arctic convoys ran in two series:
  The first series, PQ (outbound) and QP (homebound), ran from September 1941 to September 1942. These convoys ran twice monthly, with interruptions in the summer of 1942, when the series was suspended after the disaster of Convoy PQ 17, and again in the autumn after the final convoy of the series, Convoy PQ 18, because of the long daylight hours and the preparations for November 1942's Operation Torch.