By 1939, Hitler had aggressively expanded German territory through rearmament, remilitarization, annexation, and appeasement. When Hitler then turned his attention to Poland, the UK pledged support for Poland. In August 1939, the Soviet Union unexpectedly agreed to a non-aggression pact with Germany, allowing the two countries to invade and divide Poland between them. On September 1st, Germany invaded Poland, prompting Britain and its allies to declare war on Germany on September 3rd, marking the start of World War II.