Sept 3, 1939

Day 3 Sunday September 3, 1939 
The German advance into Poland continues but the main action on this day is diplomatic. Honoring their promise to protect Poland if invaded, Britain issues an ultimatum to Germany at 9 AM and declares war 2 hours later. France, Australia, New Zealand and India follow suit. Germany does not expect this intervention, after British and French appeasement following German annexation of Austria and occupation of Czechoslovakia in the previous 18 months. On hearing of the British ultimatum, Hitler asks his Foreign Minister Ribbontrop “'What now?"

KEY FIGURES Winston Churchill is appointed to Chamberlain's War Cabinet as Lord of the Admiralty. Churchill has long warned of German aggression but has been kept at bay to appease Hitler. The Admiralty signals all ships and naval bases: "WINSTON IS BACK." 

Britain declares a naval blockade of Germany. The German sub U30 sinks the British passenger ship Athenia, mistaking her for a cargo ship. 122 lives are lost.

From another site:

Atlantic Ocean

• German submarine U-30 torpedoed British passenger liner Athenia in the Atlantic Ocean. 

Austria

• Otto Skorzeny was sent home from Trost Barracks, Vienna, Austria despite the outbreak of war due to the lack of instructors to train new recruits. 

Belgium

• The government of Belgium announced their neutral stance in the European War. 

Germany

• At 0900 hours, British Ambassador in Germany Nevile Henderson delivered the British declaration of war to German Foreign Minister Joachim von Ribbentrop, effective at 1100 hours; British Commonwealth nations of New Zealand and Australia followed suit. France would also declare war later on this day, effective at 1700 hours. In the afternoon, Adolf Hitler issued an order to his generals, again stressing that German troops must not attack British and French positions. Finally, Hitler also sent a message to the Soviet Union, asking the Soviets to jointly invade Poland. 
• German government issued orders that executions by members of the SS were to be carried out in concentration camps, effective 20 Sep 1939. 
• The first RAF operation flight over Germany was a reconnaissance mission, but later in the night RAF bombers would conduct a leaflet raid. 
• Adolf Hitler departed Berlin, Germany for the Eastern Front. 

Poland

• 55 Polish peasants at Truskolasy, Poland were executed. 

German Vehicles in Poland, photo dated 9/3/1939. (Caption notes officer in SdKfz might be Guderian)

United Kingdom

• British General Edmund Ironside became the Chief of the Imperial Staff. 
• British Member of Parliament Winston Churchill became the First Lord of the Admiralty, a post he had held during the Great War
• The British government announced the immediate closure of all theatres, cinemas, music halls and other places of public entertainment. Football matches and other events likely to be attended by large crowds were banned, as was the blowing of whistles or sounding of horns.