declare war if Polish independence came under threat, as presented in a statement to the House of Commons by the British Prime Minister Neville Chamberlain on 31 March 1939 (formalized by the British on 6 April 1939; not ratified until 4 September 1939 by the French): After the declaration of war The Royal Navy initiated a naval blockade of Germany on 4 September. Although Britain and France honoured these guarantees by declaring war two days after Germany's invasion of Poland on 1 September 1939, and the dominions of the British Empire quickly followed suit, so little practical assistance was given to Poland, which was soon defeated, that in its early stages the war declared by Britain and France was described as a "Phoney War". 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. See also Canadian declaration of war on Germany Declarations of war by Great Britain and the United Kingdom Declarations of war during World War II Diplomatic history of World War II French declaration of war on Germany (1939) Military history of the United Kingdom during World War II United Kingdom declaration of war on Japan References Declarations of war during World War II France in World War II United Kingdom in World War II British Empire in World War II September 1939 events France–Germany military relations Germany–United Kingdom military relations France–United Kingdom military relations 1939 documents Germany in World War II