This is a timeline of World War II events that took place in 1940, the first full year of the second global war of the 20th century.
January
1 January: 10,000 Japanese troops launch a counter-attack in eastern Shanxi Province in China in an attempt to relieve the nearly-surrounded Japanese 36th Division.
2 January: The Soviet offensive in Finland is halted by several Finnish victories; numerous Soviet tanks are destroyed.
7 January Rationing of basic foodstuffs is established in the UK. A major Finnish victory at Suomussalmi is reported; one whole Soviet division is eliminated, and again numbers of military vehicles are captured. General Semyon Timoshenko takes command of Soviet Army forces in Finland.
10 January: Mechelen incident: a German plane, carrying plans for Fall Gelb, crashes in neutral Belgium.
16 January: Captured documents reveal Hitler's plans for the invasion of Scandinavia and a postponement of the invasion of France and the Low Countries until the spring, when the weather is more compatible for an invasion.
17 January: The Soviets are driven back in Finland and retaliate with heavy air attacks.
20 January: German submarine U-44 torpedoes and sinks Greek steamer Ekatontarchos Dracoulis off Portugal at 04:15, killing 6. U-44 had been hunting for Ekatontarchos Dracoulis for the past 6 hours.
21 January: A U-boat sinks British destroyer and its crew of 135 are all lost.
24 January: Reinhard Heydrich is appointed by Göring for the solution to the "Jewish Question".
27 January: Germany makes final plans for the invasion of Denmark and Norway.
February
1 February: The Japanese Diet announces a record high budget with over half its expenditures being military.
5 February: Britain and France decide to intervene in Norway to cut off the iron ore trade in anticipation of an expected German occupation and ostensibly to open a route to assist Finland. The operation is scheduled to start about March 20.
9 February: Erich von Manstein is placed in command of German XXXVIII (38) Armour Corps, removing him from planning the French invasion.
10 February: USSR agrees to supply grain and raw materials to Germany in a new trade treaty.
14 February: British government calls for volunteers to fight in Finland.
15 February The Soviet army captures Summa, an important defence point in Finland, thereby breaking through the Mannerheim Line. Hitler orders unrestricted submarine warfare.
16 February: British destroyer forcibly removes 303 British POWs from the German transport Altmark in neutral Norwegian territorial waters, sparking the Altmark Incident.
17 February The Finns continue retreat from the Mannerheim Line. Manstein presents to Hitler his plans for invading France via the Ardennes forest.
21 February: General Nikolaus von Falkenhorst is placed in command of the upcoming German invasion of Norway.
March
1 March: Adolf Hitler directs his generals in planning the invasion of Denmark and Norway.
3 March: Soviets begin attacks on Viipuri, Finland's second largest city.
5 March: Finland tells the Soviets they will agree to their terms for ending the war. The next day they send emissaries to Moscow to negotiate a peace treaty.
11 March: Meat rationing starts in Britain.
12 March: In Moscow, Finland signs a peace treaty with the Soviet Union after 105 days of conflict. The Finns are forced to give up significant territory in exchange for peace.
16 March: A German air raid on Scapa Flow causes the first British civilian casualties.
18 March: Hitler and Benito Mussolini meet at the Brenner pass on the Austrian border; Mussolini agrees with Hitler that Italy will enter the war "at an opportune moment".
21 March: Paul Reynaud becomes Prime Minister of France following Daladier's resignation the previous day.
28 March: Britain and France make a formal agreement that neither country will seek a separate peace with Germany.
29 March: The Soviets want new territories. Molotov speaks to the Supreme Soviet, about "an unsettled dispute", the question of Romanian Bessarabia.
30 March Japan establishes a puppet regime at Nanking, China, under Wang Jingwei. Britain undertakes secret reconnaissance flights to photograph the targeted areas inside the Soviet Union in preparation for Operation Pike, utilising high-altitude, high-speed stereoscopic photography pioneered by Sidney Cotton.
April
3 April 22,000 Polish officers, policemen, and others are massacred by the Soviet NKVD in the Katyn massacre. The Ministerial Defence Committee, with the First Lord of the Admiralty (Winston Churchill) as its chair, replaces Lord Chatfield's ministerial position of Minister for Coordination of Defence.
9 April: Germans land in several Norwegian ports and take Oslo; the Norwegian Campaign lasts two months. The British start their Norwegian Campaign. Denmark is invaded and surrenders in six hours. The German heavy cruiser Blücher is sunk at the Battle of Drøbak Sound.
10 April Germans set up a Norwegian government under Vidkun Quisling, former minister of defence. The German light cruiser Königsberg is sunk by British Fleet Air Arm dive bombers.
11 April: First Battle of Narvik. British destroyers and aircraft successfully make a surprise attack against a larger German naval force. A second attack on April 13 will also be a British success.
12 April: British troops occupy the Danish Faroe Islands.
14 April: British and French troops start landing at Namsos, north of Trondheim in Norway.
15 April: British troops land at Harstad, near Narvik, Norway.
16 April: More British landings in Norway, notably north and south of Trondheim; the struggle for Trondheim continues until the 22nd.
27 April: British troops start pull-out from central Norway, north and south of Trondheim.
May
1 May: Allies begin evacuating Norwegian ports; the efforts will continue until June.
5 May: Norwegian government in exile established in London.
7 May: The Norway Debate begins in the House of Commons.
8 May: Prime Minister Neville Chamberlain barely survives a confidence vote in the House of Commons.
9 May: Conscription in Britain extended to age 36.
10 May Germany invades France and The Benelux; Winston Churchill becomes Prime Minister of the United Kingdom upon the resignation of Neville Chamberlain. The United Kingdom invades Iceland. Belgium declares a state of emergency. Churchill is called on to form a wartime coalition government. The massive German offensive against the Western front: The invasion of Luxembourg, Belgium, the Netherlands, and France begins. In a bold stroke, German paratroops capture the Belgian fort Eben Emael. The Battle for The Hague becomes the first failed paratrooper attack in history as the Dutch quickly defeat the invaders.
11 May Luxembourg is occupied by the Germans. Churchill offers the former Kaiser Wilhelm II, who is now living in the Netherlands, asylum in the United Kingdom; he declines.
12 May The Belgians blow up all the bridges over the Meuse River to halt the German advance. Battle of Hannut begins in Belgium.
13 May Dutch government-in-exile established in London. General Heinz Guderian's Panzer corps breaks through at Sedan, France. Queen Wilhelmina of the Netherlands flees to asylum in the United Kingdom. Churchill's "blood, toil, tears, and sweat" speech in Commons. The Dutch lose the Battle of the Grebbeberg to the Germans.
14 May The creation of the Local Defence Volunteers (the Home Guard) is announced by the new Secretary of State for War Anthony Eden. It is mostly composed of the elderly and retired. Churchill asks President Roosevelt and Canada for aid in these dark days. Outlines of the new British coalition, which includes Labour, Liberal, and Conservative members, is made public. The Dutch defeat the Germans at the Battle of the Afsluitdijk. The Rotterdam Blitz led to German success in the Battle of Rotterdam, while causing many civilian deaths and tremendous damage. The Netherlands decides to surrender with the exception of Zeeland.
15 May The capitulation of the Dutch army is signed. In a response to the Rotterdam Blitz, the first large-scale strategic bombing of World War II targets Gelsenkirchen, followed by Hamburg, Bremen, Cologne, Essen, Duisburg, Düsseldorf and Hanover during the next days. German forces cross over the Meuse River.
16 May Churchill visits Paris and hears that the French war is as good as over. The Belgian government leaves Belgium for Bordeaux in France, as the Belgian army retreats. It later moves to London.
17 May Germans enter Brussels and also take Antwerp. Paul Reynaud forms new French government, including 84-year-old Marshal Pétain, the French hero of World War I.
18 May Maxime Weygand replaces Maurice Gamelin as commander of the French armed forces. Antwerp captured. The Germans win the Battle of Zeeland.
19 May Amiens in France is besieged by German troops; Rommel's forces surround Arras; other German forces reach Noyelles on the Channel. The British complete their invasion of Iceland.
20 May: General Guderian's Panzer groups take Abbeville, threatening Allied forces in the area.
23 May: Oswald Mosley, leader of the pre-war British fascists, is jailed; he and his wife will spend the war’s duration in prison.
24 May: The British make a final decision to cease operations in Norway.
25 May The Allied forces, British and French alike, retreat to Dunkirk. Hitler orders a halt to the advance of Germans toward the Allied beachhead and allows Hermann Göring to use the Luftwaffe to attack. British R.A.F. defends the beachhead. Sporadic Luftwaffe bombings in England. Boulogne-sur-Mer surrenders to the Germans. Soviet Union is preparing a total takeover in the Baltic States organizing and staging conflicts between the Baltic States and the USSR. Soviet government accuses Lithuania of kidnapping Soviet soldiers.
25-28 May: 86 Belgian civilians are murdered by German forces in the village of Vinkt.
26 May The Patrol vessel A4 arrives in Plymouth, evacuating the final 40 tonnes of national gold reserves out of Belgium. Calais surrenders to the Germans. Operation Dynamo, the Allied evacuation of 340,000 troops from Dunkirk, begins. The move will last until June 3 under ferocious bombardment by the Luftwaffe.
28 May: Belgium surrenders to the Germans; King Leopold III of Belgium surrenders and is interned.
30 May: Crucial British Cabinet meeting: Churchill wins a vote on continuing the war, in spite of vigorous arguments by Lord Halifax and Chamberlain.
31 May: The Japanese heavily bomb the Chinese Nationalist capital of Chongqing, on the upper Yangtze.
June
3 June Last day of Operation Dynamo. 224,686 British and 121,445 French and Belgian troops have been evacuated. Germans bomb Paris.
4 June: Winston Churchill delivers his, "We shall fight on the beaches...we shall never surrender" speech to the House of Commons.
7 June: German battleships Gneisenau and Scharnhorst sink the aircraft carrier HMS Glorious and two destroyers off Norway; the British ships have had no air cover.
9 June: Red Army provokes conflicts on the Latvian border.
10 June: Italy declares war on France and the United Kingdom. Norway surrenders after 2 months of combat, the longest of any nation that had been invaded by Germany. King Haakon and his government had evacuated to Britain three days previously. French government moves to Tours.
11 June: The Siege of Malta begins.
12 June: More than 10,000 British soldiers of the 51st (Highland) Division are captured at Saint-Valéry-en-Caux.
13 June: The French government moves again, this time to Bordeaux.
14 June Paris occupied by German troops; Elements of the French Navy (Marine Nationale) based in Toulon carried out offensive operations against Italian targets along the Ligurian coast.