World War II (Prelude to): In violation of the Locarno Pact and the Treaty of Versailles, Germany reoccupies the Rhineland.

Chinese Air Force Day: The beginning of air-to-air combat of the Second Sino-Japanese War and World War II in general, when 6 Imperial Japanese Mitsubishi G3M bombers are shot down by the Nationalist Chinese Air Force while raiding Chinese air bases.

World War II: Germany takes Memel from Lithuania.

World War II: Italy invades Albania.

World War II: Germany and Italy sign the Pact of Steel.

World War II: Germany and the Soviet Union sign a non-aggression treaty, the Molotov-Ribbentrop Pact. In a secret addition to the pact, the Baltic states, Finland, Romania, and Poland are divided between the two nations.

World War II: Nazi Germany invades Poland, beginning the war in Europe.

World War II: following the start of the invasion of Poland the previous day, the Free City of Danzig (now Gdańsk, Poland) is annexed by Nazi Germany.

World War II: France, the United Kingdom, New Zealand and Australia declare war on Germany after the invasion of Poland, forming the Allies.

World War II: The Battle of Barking Creek.

World War II: The submarine HMS Oxley is mistakenly sunk by the submarine HMS Triton near Norway and becomes the Royal Navy's first loss.

World War II: The Soviet Union joins Nazi Germany's invasion of Poland during the Polish Defensive War of 1939.

World War II: Polish government of Ignacy Mościcki flees to Romania.

Nazi Germany and the Soviet Union agree on a division of Poland after their invasion during World War II.

World War II: The last Polish army is defeated.

World War II: Germany annexes Western Poland.

World War II: First attack on British territory by the German Luftwaffe.

World War II: U.S. President Franklin D. Roosevelt orders the United States Customs Service to implement the Neutrality Act of 1939, allowing cash-and-carry purchases of weapons by belligerents.

World War II: Sonderaktion Krakau takes place.

World War II: ''HMS Nelson'' is struck by a mine (laid by ''U-31'') off the Scottish coast and is laid up for repairs until August 1940.

World War II: Battle of the River Plate

World War II: Battle of the River Plate

World War II: Pope Pius XII makes a Christmas Eve appeal for peace.

World War II: Britain introduces food rationing.

World War II: Altmark Incident: The German tanker Altmark is boarded by sailors from the British destroyer HMS Cossack. 299 British prisoners are freed.

In a ceremony held in Berkeley, California, because of the war, physicist Ernest Lawrence receives the 1939 Nobel Prize in Physics from Sweden's Consul General in San Francisco.

World War II: Axis Powers

World War II: The United Kingdom and France announce that they have mined Norwegian territorial waters to prevent their use by German supply ships.

World War II: Operation Weserübung

World War II: Royal Marines land in Namsos, Norway in preparation for a larger force to arrive two days later.

World War II: Norwegian refugees form a government-in-exile in London

World War II: The German submarine ''U-9'' sinks the French coastal submarine Doris near Den Helder.

World War II: The first German bombs of the war fall on England at Chilham and Petham, in Kent.

World War II: Germany's conquest of France begins as the German army crosses the Meuse. Winston Churchill makes his "blood, toil, tears, and sweat" speech to the House of Commons.

World War II: Rotterdam is bombed by the German Luftwaffe.

World War II: After fierce fighting, the poorly trained and equipped Dutch troops surrender to Germany, marking the beginning of five years of occupation.

World War II: Germany occupies Brussels, Belgium.

World War II: The Battle of Dunkirk begins.

World War II: Battle of Dunkirk

World War II: In the Le Paradis massacre, 99 soldiers from a Royal Norfolk Regiment unit are shot after surrendering to German troops. Two survive.