Britain and France entered into a formal military alliance with Poland on August 25, 1939, in response to the escalating tensions between Germany and Poland. The alliance was an attempt to deter Nazi Germany from further aggression against Poland and to demonstrate the commitment of Britain and France to collective security in Europe. However, despite their alliance with Poland, Britain and France failed to prevent the German invasion of Poland on September 1, 1939, which marked the beginning of World War II.