* Declared war on Germany on 3 September 1939. This was in accordance with their mutual assistance pact with Poland.
* Began the Phoney War, a period of limited military activity along the Franco-German border.
* Sent the British Expeditionary Force (BEF) to France to help defend the country against a German invasion.
* Blockaded Germany to prevent it from importing essential supplies.
United States
* Condemned the German invasion of Poland.
* Imposed economic sanctions on Germany.
* Increased its military spending.
* Passed the Neutrality Act of 1939, which prohibited the sale of arms and munitions to belligerent countries.
* Gave moral support to Britain and France.
* Offered asylum to Jewish refugees from Europe
Soviet Union
* Invaded Poland from the east on 17 September 1939, in accordance with the secret Molotov-Ribbentrop Pact with Germany.
* Occupied eastern Poland and annexed it to the Soviet Union.
* Signed a non-aggression pact with Germany on 23 August 1939, which helped to pave the way for the German invasion of Poland.
* Collaborated with the Germans during the initial stages of the war