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). The Polish ambassador in London, Edward Bernard Raczyński, contacted the British Foreign Office to point out that clause 1(b) of the agreement, which concerned an "aggression by a European power" on Poland, should apply to the Soviet invasion. Foreign Secretary Lord Halifax responded that the obligation of British Government towards Poland arising out of the Anglo-Polish Agreement was restricted to Germany, according to the first clause of the secret protocol. The Soviet Union held sway over the former Polish territory at the war's conclusion, having become a part of the Allies 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.