In 1939, France and Great Britain declared war on Germany as a direct result of Germany invading Poland. Known as September Campaign in Poland and Poland Campaign in Germany, the invasion happened on September 1, 1939 which was followed by the Soviet Union invading the Poland on September 17, 1939. After failing to comply to Sir Henderson's (Great Britain's ambassador to Germany) ultimatum to cease the invasion, the Great Britain declared war on Germany with France following.
