1940 was a leap year.
February 16 - Sailors from the British destroyer HMS Kozak board the German tanker Altmark and release 299 British prisoners of war.
March 12 - After the defeat in the Winter War, Finland signs the Moscow Peace Agreement with the Soviet Union, leaving almost all of Karelia to him.
April 9 - German troops attack Denmark and Norway in World War II in Operation Weseribung.
May 10 - Germany invades the Netherlands, Belgium and Luxembourg without declaring war, and Britain occupies the former Danish colony of Iceland after the German invasion of Denmark.
May 10 - Winston Churchill is elected Prime Minister of the United Kingdom in place of Neville Chamberlain.
May 14 - During negotiations to surrender the Dutch army in World War II, German bombers destroy two-thirds of Rotterdam, killing about a thousand people and leaving 80,000 homeless.
May 21 - Franco-British forces counterattack at the Battle of Aras slows German advances in northeastern France.
June 3 - The evacuation of more than 337,000 British, French and Belgian troops from Dunkirk during the Battle of France is completed.
June 8 - German cruisers Scharnhorst and Gneissenau sink the British aircraft carrier Glorius and the destroyers Ardent and Akasta in World War II, killing more than 1,500 people.
June 14 - Germans capture Paris during the Battle of France.
June 17 - The Soviet Red Army occupies Latvia and Estonia and establishes a pro-Soviet administration.
June 17 - The Luftwaffe sinks HMT Lancaster near Saint-Nazaire, killing about 3,000 people.
June 18 - French General Charles de Gaulle from London sends a radio message to the French in which he declares himself the leader of Free France, and calls on the French to resist the German occupation.
June 22 - France signs capitulation in Compiègne in World War II.
July 3 - The British disable a large part of the French fleet stationed in Mers-el-Kebir, Algeria, in order not to come into German possession.
July 9 - Battle of Calabria
July 10 - The battle for Britain in World War II begins with the attack of German bombers on British convoys in Lithuania.