Anti-Soviet rumblings began shortly after the Red Army seized eastern Poland in September 1939, and became more strident after Joseph Stalin invaded Finland later that year. Truly serious planning did not take place, however, until after Stalin's disastrous war with Finland ended in March 1940. Immediately following the abrupt end of that conflict the Red Army sent substantial reinforcements to the Black Sea region, the area most threatened by Allied attack. In March-April 1940, the British undertook secret reconnaissance flights to obtain photographs of important targets inside the Soviet Union. The swift collapse of France in May 1940 insured that British bombers were not launched against these targets, but suspicion lingered between Britain and the USSR throughout the war, contributing to Stalin's refusal to believe Winston Churchill's warnings that Hitler was preparing to invade the USSR in 1941.