Late 1939–March 12, 1940 The Soviet Union invades Finland, initiating the so-called Winter War. The Finns sue for an armistice and cede the northern shores of Lake Lagoda to the Soviet Union. They also cede the small Finnish coastline on the Arctic Sea.

April 9, 1940–June 9, 1940 Germany invades Denmark and Norway. Denmark surrenders on the day of the attack. Norway holds out until June 9.

May 10, 1940–June 22, 1940 Germany attacks western Europe specifically France and the neutral Low Countries (Belgium, the Netherlands, and Luxembourg). Luxembourg is occupied on May 10; the Netherlands surrenders on May 14; and Belgium surrenders on May 28. On June 22, France signs an armistice agreement by which the Germans occupy the northern half of the country and the entire Atlantic coastline. In southern France, a collaborationist regime with its capital in Vichy is established.

June 10, 1940 Italy enters the war. Italy invades southern France on June 21.

June 28, 1940 The Soviet Union forces Romania to cede the eastern province of Bessarabia and the northern half of Bukovina to Soviet Ukraine.

June 14, 1940–August 6, 1940 The Soviet Union occupies the Baltic states (Estonia, Latvia, and Lithuania) on June 14–18. On July 14–15, it engineers Communist coup d’états in each of these countries and then annexes them as Soviet Republics on August 3–6.

July 10, 1940–October 31, 1940 The air war known as the Battle of Britain ends in defeat for Nazi Germany.

September 13, 1940 The Italians invade British-controlled Egypt from Italian-controlled Libya.

September 27, 1940 Germany, Italy, and Japan sign the Tripartite Pact.

October 1940 Italy invades Greece from Albania on October 28.

November 1940 Hungary (November 20), Romania (November 23), and Slovakia (November 24) join the Axis.