At the outset of the Soviet invasion of Poland (17 September 1939), the Soviets declared that the Polish state and government—as a result of the German invasion of Poland that began on 1 September 1939—no longer existed and proclaimed any treaty or diplomatic relations between the Soviet Union and Poland invalid. Diplomatic relations were re-established in 1941 after the German invasion of the Soviet Union, when the British government allied itself with the attacked Soviet Union and pressured the Polish government to act accordingly. Consequently, the military agreement of 14 August and the Sikorski–Mayski agreement of 17 August, between the Polish government-in-exile and the Soviet government, were signed; Joseph Stalin agreed to abrogate the Poland-related aspects of the Molotov–Ribbentrop Pact. As an amnesty for Polish citizens in the Soviet Union was negotiated, tens of thousands of Polish prisoners of war held in Soviet camps, as well as hundreds of thousands of Polish citizens who had been deported to the USSR, were released.