19391940

1939–1940 denotes the early phase of World War II, when a regional conflict rapidly expanded into a continental and then global war. The period covers the outbreak of hostilities in September 1939 following Germany’s invasion of Poland, the ensuing declarations of war by Britain and France, and the broader winter and spring campaigns that shaped the European theater. It also encompasses the Soviet Union’s actions in the region and the initial clashes that foreshadowed a long war across multiple fronts.

In 1939, Germany invaded Poland on September 1, triggering Britain and France to declare war on September 3. The Soviet Union invaded eastern Poland on September 17, and in August the Molotov–Ribbentrop Pact between Nazi Germany and the Soviet Union laid the groundwork for the subsequent division of Poland. The Winter War began in November 1939 when the Soviet Union attacked Finland, continuing into 1940 and highlighting the coercive dimensions of the conflict.

In 1940, German militarism intensified with invasions of Denmark and Norway in April, followed by attacks on the Netherlands, Belgium, and Luxembourg in May. France and the Low Countries fell under German control in May–June, culminating in the Dunkirk evacuation (late May–early June) and the armistice with France on June 22. The period also saw the onset of the Battle of Britain, the first major military campaign fought primarily in the air, beginning in mid-1940.

The 1939–1940 interval marks a turning point from political maneuvering to sustained, large-scale warfare, establishing German strategic dominance in Western Europe, signaling broader Soviet interests, and setting the stage for a wider, protracted global conflict.