| 7 January | Finnish forces destroy the Red Army's 44th Rifle Division at Raate road. |
| 8 January | England imposes rationing on bacon, sugar, butter and other food supplies. |
| 8 January | Soviet troops surrender around Lakes Kuivasjarvi and Kuomasjarvi as Finnish 9th division occupies Raate Road. |
| 1 February | Russian forces launch a major assault on Finnish troops occupying the Karelian Isthmus. |
| 8 February | Germans establish the first large Jewish ghetto in Poland in the city of Lodz. |
| 11 February | A mass attack by 140,000 Soviet troops strikes Finnish defenses on the Karelian Isthmus, overwhelming the Finnish defense. |
| 16 February | British destroyer HMS Cossack pursues the disguised German tanker Altmark into Norwegian waters [Jøssingfjord]; boarding party finds 299 captive British seamen. |
| 25 February | First squadron of Royal Canadian Air Force [RCAF] reaches Britain to assist in the defense. |
| 26 February | Finland announces evacuation of Koivisto coastal fortress, on right flank of Mannerheim Line. |
| 29 February | Battle of Viipuri begins [Finland] as Russians attempt to crush resistance in Karelian Isthmus [29 Feb-13 Mar]. |
| 29 February | Food and gas rationing begsn in France. |
| 5 March | Soviet Politburo approves Lavrentiy Beria's proposal to execute the Polish Officer Corps, the plan that becomes the Katyn Forest Massacre. |
| 12 March | Peace signed in Moscow between USSR and Finland. |
| 18 March | Adolf Hitler and Benito Mussolini meet at Brenner Pass in the Alps [between Austria and Italy] agreeing to an alliance against France and Britain. |
| 2 April | Soldiers of the Chinese Nationalist Army reoccupy Wuyuan after successfully ambushing some 3,000 Japanese troops. |
| 3 April | Katyn Forest Massacre: In a series of events, 21,857 Polish internees and prisoners were executed by Soviet secret police on or shortly after this date. |
| 9 April | Germany occupies Denmark and invades Norway [Operation Weserübung]. |
| 9 April | British campaign in Norway is launched. |
| 12 April | To pre-empt a German invasion following the German occupation of Denmark and Norway, the British occupy the strategically important Faroe Islands [Operation Valentine]. |
| 14 April | British North Western Expeditionary Force begins landing in Norway to oppose German invasion. |
| 30 April | Two killed on the ground in Clacton-on-Sea by the crash of a German Heinkel aircraft, England's first civilian casualties of the war. |
| 10 May | German Blitzkrieg ["lightning war"] begins with air attacks on Dutch cities and invasion of Belgium and Luxembourg. |
| 10 May | British Prime Minister Chamberlain resigns and is replaced by Winston Churchill. |
| 10 May | Iceland invaded by the United Kingdom [Operation Fork] to prevent a German presence there. |
| 11 May | Belgian fortress Eben Emael, on the Meuse River, reputed to be the most formidable stronghold in the world, falls to German Army glider assault. |
| 11 May | Luxembourg falls to the German army. |
| 11 May | On the night of 11-12 May, 1940 the Royal Air Force [RAF] attacked Mönchengladbach, Germany, the first Allied air raid on Germany of WW II. |
| 13 May | Winston Churchill addresses Parliment for the first time as Prime Minister, saying, "I have nothing to offer you but blood, toil, tears, and sweat". |
| 13 May | French Maginot Line is breached at Sedan; General Guderian's 19th Panzer Corps pours through a 60-mile (97km) wide gap. |
| 14 May | 980 killed and 20,000 buildings destroyed in Rotterdam by brutal Luftwaffe bombing; Dutch government flees to London. |
| 15 May | Dutch armies surrender to Germany. |
| 16 May | US Pres. Roosevelt asks Congress for funding to build 50,000 airplanes per year, a startling undertaking at the time. |
| 17 May | Brussels falls to German forces; Belgian government flees to Ostend, Belgium. |
| 20 May | German General Rudolf Veiel's 2nd Panzer division reaches the English Channel at Noyelles, splitting the Allied defense. |
| 20 May | Largest Nazi concentration camp, Auschwitz-Birkenau, opens in Poland. Before liberation in 1945, about 1.1 million people died there, 90 percent Jews. |
| 25 May | Boulogne, France is captured by the Germans. |
| 26 May | Evacuation of Dunkirk begins, withdrawing British forces from continental Europe [Operation Dynamo, 26 May-3 June]. |
| 28 May | Belgium surrenders to Germany. |
| 2 June | Adolf Hitler makes a tour of the North of France, meeting with German military leaders and visiting places where he served in WW I. |
| 3 June | Civilians flee the first bombing of Paris by German planes, as Germany prepares for the invasion of France. |
| 4 June | Neuengamme concentration camp, near Hamburg, Germany becomes autonomous, no longer a sub-camp of Sachsenhausen. |
| 5 June | 100 Wehrmacht divisions with 2000 Panzers plus massive air support attack France at four points, outflanking the static Maginot defenses and overwhelming the pathetic Weygand Line. |
| 8 June | British Navy fully withdrawn from Norway after ineffective defense operations against German invasion. |
| 9 June | British Commandos are created as a new type of military unit, tasked to show Germany that Britain was not out of the fight. |
| 10 June | Mussolini's fascist Italy declares war on Britain and France, invading through the Riviera with 400,000 troops. |
| 10 June | Norway surrenders to Germany. |
| 11 June | French government flees to Tours, abandoning Paris to the Germans. |
| 12 June | 13,000 British and French troops surrender to Field Marshal Erwin Rommel at St. Valery-en-Caux, France. |
| 13 June | Paris, France declared an open city as German Army closes in. |
| 14 June | German army seizes Paris as French government flees to Bordeaux. |
| 14 June | Auschwitz concentration camp receives 728 Polish political prisoners from Tarnòw, Poland, the first residents of the camp. |
| 15 June | Soviet Red Army occupies Baltic countries: Lithuania, Latvia, Estonia [June 15-16]. |
| 15 June | Verdun falls to German advamce. |
| 16 June | Armistice advocate, 84-year-old Marshal Henri Philippe Pétain becomes Prime Minister of France, requests peace terms from Germany, forms Vichy France government. |
| 17 June | As Germany overruns France, Allied troops execute Operation Ariel, the evacuation of France [15-25 June]. |
| 17 June | Luftwaffe bombs and sinks the British ship RMS Lancastria with the loss of 5800 troops being evacuated near Saint-Nazaire, France. |
| 18 June | General Charles de Gaulle broadcasts from London, calling on all French people to continue the fight against Nazi Germany. |
| 18 June | Winston Churchill speaks to Parliament, concluding, "...if the British Empire and its Commonwealth last for a thousand years, men will still say, 'This was their finest hour'". |
| 22 June | Vichy France signs armistice with Germany, staged at Compiegne, in the same railway car where Germany surrendered to the Allies in 1918, at the end of WW I. |
| 22 June | Army sub-committee of the US Ordnance Department's Technical Committee, issued proposed specifications for a new lightweight vehicle that was to become the first military jeep. |
| 23 June | Adolf Hitler visits newly-defeated Paris to savor his victory over France. |
| 24 June | Vichy France government signs armistice with Italy. |
| 24 June | Newly formed British Commando unit stages first cross-Channel raid with 4 RAF air-sea rescue boats. Little was gained from the raid. |
| 27 June | Rumania cedes Bessarabia and No. Bukovina to USSR. |
| 28 June | General Charles de Gaulle officially recognized by Britain as the leader of all French people. |
| 30 June | German forces land in Guernsey, the start of their five year Occupation of the Channel Islands. |
| 3 July | British shell French ships at Oran, Algeria. |
| 8 July | Tea rationing begins in Britain. |
| 9 July | Battle of Calabria [Battle of Punta Stilo, the toe of Italy] fought between the Italian Royal Navy and the British/Australian Royal Navies, 30 miles [48km] east of Punta Stilo. |
| 10 July | Agreement divides France into German occupied zone of northern and western France, including the entire Atlantic coast, and unoccupied zone, the remaining 2/5 of France, under Pétain at Vichy. |
| 10 July | Germain air campaign against England, the Battle of Britain, begins with German attacks on English Channel naval targets [10 Jul-31 Oct]. |
| 11 July | US Quartermaster Corps asked 135 companies to bid on seventy light reconnaissance and command cars, the origin of the military jeep. |
| 23 July | American Bantam and Willys-Overland submit the only two bids for the Quartermaster Corps light reconnaissance and command cars, the origin of the military jeep. |
| 30 July | President of Estonia, Konstantin Päts, is imprisoned by Soviet agents and deported to Russia. |
| 10 August | Britain takes decision to deploy almost all of its tanks to North Africa, despite still-looming German invasion potential. |
| 15 August | Peak day for Luftwaffe sorties against England, including bombing in the North of Britain. |
| 17 August | Italian armed forces under Marshal d'Armata Rodolfo Graziani invaded and occupied British Somaliland, threatening American Red Sea merchant shipping and British access to India. |
| 20 August | Winston Churchill refers to the valiant RAF pilots defending Britain: 'Never in the field of human conflict was so much owed by so many to so few'. |
| 25 August | RAF Bomber Command mounts a risky but successful reprisal raid on Berlin, a shock to German leadership who had been assured by Göring that such a raid was impossible [25-26 Aug]. |
| 26 August | No. 1 Fighter Squadron, Royal Canadian Air Force [RCAF] engages German bombers over southern England, the first RCAF combat. |
| 2 September | US-Great Britain Destroyers for Bases agreement, transferred 50 mothballed US destroyers to UK in exchange for land rights on British possessions. |
| 7 September | First day of the London Blitz, followed by German bombing for 57 consecutive nights over London and other British cities. |
| 8 September | Edward R. Murrow broadcasts from London with live reports of the Blitz, increasing America's understanding of the war between Hitler and isolated Britain. |
| 13 September | Italian Marshall Rodolfo Graziani begins an eastward advance from Libya into Egypt, with five divisions headed for the British controlled Suez Canal. |
| 16 September | Selective Training and Service Act of 1940 signed by Pres. Roosevelt, beginning the military draft of US men aged 21-36. |
| 23 September | After just seven weeks of development, American Bantam delivers the first prototype jeep to Camp Holabird, MD. |
| 26 September | Japanese troops land at Dong Tac, south of Haiphong port in French Indo-China [now Vietnam], advance to seize the harbor, airport and other facilities. |
| 27 September | Germany, Italy and Japan sign Tripartite Pact. |
| 4 October | Adolf Hitler and Benito Mussolini meet for conference at Brenner Pass in Alps, with staff including their foreign secretaries. |
| 7 October | First German troops, leading to an eventual one-half million, enter Romania [7-8 Oct]. |
| 9 October | London's St. Paul's Cathedral hit by a bomb during a Luftwaffe Blitz nighttime attack [9-10 Oct]. |
| 15 October | In the heaviest attacks of the Blitz so far, Birmingham and Bristol suffer while 400 bombers hit London for six hours. Exhausted RAF puts up only 41 fighters, shooting down only one bomber. |
| 16 October | Selective Service draft registration begins in the United States for approximately 16 million men. |
| 16 October | 34 ships sunk from Convoy SC-7 and Convoy HX-79 by German submarine Wolf Pack, one of the worst attacks of the war in the Atlantic [16-19 Oct]. |
| 26 October | First flight of the North American Aviation P-51 Mustang aircraft, only 117 days after the Government contract was awarded. |
| 28 October | Greek Prime Minister Ionannis Metaxas refuses demand from Italian dictator Benito Mussolini to permit Italian troops to occupy Greece. |
| 28 October | Balkans Campaign begins as Italian troops invade Greece, meeting strong resistance from Greek troops and civilians. |
| 29 October | First Selective Service System draft lottery is held in Washington, DC; Number 158 is the first chosen. |
| 31 October | End of Battle of Britain, Hitler's air war against London, other British cities, and the Royal Air Force [RAF]. |
| 5 November | President Franklin Roosevelt elected to an unprecedented third term. |
| 7 November | RAF Bomber Command raids Krupp munitions factory in Essen, Germany. |
| 11 November | 55 Polish intellectuals murdered in Dachau concentration camp [near Munich], the first of many mass executions there. |
| 11 November | Operation SWORDFISH: Raid on Taranto, Italy, the British Royal Navy's first all-aircraft naval attack, launching obsolete canvas planes from an aircraft carrier [11-12 Nov]. |
| 13 November | Willys delivered the Quad prototype jeep to the US Army. |
| 14 November | German Blitz air raids on Coventry, England with 449 bombers, destroy much of the city and its cathedral [November 14-16]. |
| 14 November | Greek army pushes the Italians out of Greece, chasing them into Albania. |
| 16 November | RAF Bomber Command raids Hamburg, Germany. |
| 18 November | Alfred Loomis suggests the electronic air navigation system which was later developed into LORAN [long range navigation system] at MIT. |
| 20 November | Hungary joins Hitler's Axis Tripartite Pact, followed by Romania and Slovakia in the next few days [November 20-25]. |
| 23 November | Ford delivered the Pygmy prototype jeep to the US Army. |
| 9 December | British North African offensive begins against Italian forces in Somaliland, Egypt and Libya. |
| 18 December | The Chief of Naval Operations authorizes use of the acronym 'RADAR' derived from the term Radio Detection And Ranging. |
| 25 December | Admiral Darlan and Hitler meet to solidify cooperation between Germany and Vichy France government. |
| 25 December | British Royal Navy earns the first kill for a Grumman F4F Wildcat, shooting down a Junkers Ju 88. |
| 29 December | London hit by the most devastating raid of the Blitz, causing destruction referred to as 'The Second Great Fire of London'. |
