The primary reason for Great Britain's entry into World War II was the invasion of Poland by Nazi Germany on September 1, 1939. Britain had a mutual defense pact with Poland, which obligated it to come to Poland's aid in the event of war. The British government, led by Prime Minister Neville Chamberlain, had been pursuing a policy of appeasement towards Nazi Germany in the years leading up to the war, in the hope of avoiding a conflict. However, the invasion of Poland was seen as a step too far, and Britain declared war on Germany on September 3, 1939.