1939
September
- 1 Sep — Invasion of Poland by Germany; Norway, Switzerland and Finland declare neutrality
- 3 Sep — United Kingdom, Australia, New Zealand and France declare war on Germany
- 17 Sep — Soviet Union invades Eastern Poland
- 28 Sep — Warsaw surrenders
November–December
- 30 Nov — Soviet Union invades Finland
- 13 Dec — Battle of the River Plate
- 17 Dec — Admiral Graf Spee scuttled in Montevideo harbour
1940
- 9 Apr — Denmark and Norway invaded by Germany; Denmark surrenders
- 10 May — Belgium, France, Luxembourg and Netherlands invaded by Germany; Churchill becomes PM
- 26 May — Operation Dynamo (Dunkirk evacuation) begins
- 10 Jun — Italy declares war; Norway surrenders
- 22 Jun — Franco-German armistice signed
- Jul–Sep — Battle of Britain; Operation Sealion postponed
- 13 Sep — Italy invades Egypt
- 11 Nov — British naval attack on Italian fleet at Taranto
1941
- 11 Mar — Lend Lease Act signed
- 6 Apr — Germany invades Yugoslavia and Greece
- 24 May — HMS Hood sunk by Bismarck
- 27 May — Bismarck sunk by Royal Navy
- 22 Jun — Germany invades Soviet Union (Operation Barbarossa)
- 15 Sep — Siege of Leningrad begins
- 7 Dec — Japanese attack Pearl Harbor, Hong Kong, Singapore, Malaya
- 11 Dec — Germany and Italy declare war on the United States
1942
- 15 Feb — Singapore surrenders to Japanese forces
- 18 Apr — Doolittle Raid on Japan
- 4 May — Battle of the Coral Sea
- 4 Jun — American victory at Battle of Midway
- 7 Aug — Battle of Guadalcanal begins
- 23 Oct — Second Battle of El Alamein begins
- 8 Nov — Operation Torch (Allied invasion of North Africa)
- 19 Nov — Soviet counterattack at Stalingrad begins
1943
- 31 Jan — German 6th Army surrenders at Stalingrad
- 5 Jul — Battle of Kursk begins
- 9 Jul — Allied invasion of Sicily
- 3 Sep — British troops invade mainland Italy
- 23 Sep — Italy surrenders to the Allies
1944
- 18 Jan — Siege of Leningrad ends
- 6 Jun — D-Day: Operation Overlord launched in Normandy
- 25 Aug — Paris liberated
- 17 Sep — Operation Market Garden begins
- 16 Dec — Battle of the Bulge begins
1945
- 27 Jan — Auschwitz liberated by Soviet troops
- 19 Feb — US Marines invade Iwo Jima
- 1 Apr — US troops invade Okinawa
- 30 Apr — Hitler commits suicide in Berlin
- 7 May — Germany surrenders unconditionally
- 6 Aug — Atomic bomb dropped on Hiroshima
- 9 Aug — Atomic bomb dropped on Nagasaki
- 15 Aug — Japan announces unconditional surrender
- 2 Sep — Japan signs surrender on USS Missouri