Further , neither the British Empire , nor the French , ever declared war upon the Soviet Union , which invaded Poland on 17 September 1939 ( 16 days after Nazi Germany invaded from the West ) and held sway over the former-Polish territory at the war 's conclusion , having become a part of the Allied front in the course of World War II . At the insistence of Joseph Stalin , the post-war Yalta Conference in 1945 sanctioned the formation of a new provisional pro-Communist coalition government in Moscow , which ignored the Polish government-in-exile based in London
Yalta Conference
Yalta Conference
British and French declaration of war on Germany