The Polish September Campaign was the conquest of Poland by the armies of Nazi Germany, the Soviet Union, and a small contingent of Slovak forces during the Second World War. The campaign began on 1 September 1939 following a German-staged attack. This military operation, which saw the first use of Blitzkrieg tactics, marked the start of the Second World War in Europe as the invasion led Poland's allies, the United Kingdom and France, to declare war on Germany on September 3. On September 17, 1939, the Soviet Red Army invaded the eastern regions of Poland. The Soviets were acting in co-operation with Nazi Germany, carrying out their part of the secret appendix of the Molotov–Ribbentrop Pact (the division of Europe into Nazi and Soviet spheres of influence). The campaign ended on 6 October 1939, with Germany and the Soviet Union occupying the entirety of Poland.