The Warsaw Uprising
In 1944 the London Poles (so named because they had fled from Poland to London following the outbreak of WW2) orchestrated an operation to gain control of Warsaw. The planning of the attack was intended to be such that it coincided with the German retreat, but was ahead of the Soviet invasion.
The Soviet government refused to assist the London Poles in the Uprising; a direct order from the Kremlin halted the advance Soviet troops moving on Warsaw. In addition, there was a refusal to permit the Western Allies to use Soviet air bases to provide support to the London Poles. As a result of the uprising, the military force of the London Poles was considerably weakened which later reduced their ability to resist the USSR.