1940: The war spreads

Europe

Germany invaded Denmark and Norway on 9 April 1940 in Operation Weserübung, in part to counter the threat of an impending Allied invasion of Norway. Denmark did not resist, but Norway fought back, and was joined by British, French, and Polish (exile) forces landing in support of the Norwegians at Namsos, Åndalsnes, and Narvik. By late June the Allies were defeated, German forces were in control of most of Norway, and what remained of the Norwegian Army had surrendered.

On 10 May 1940 the Germans invaded Luxembourg, Belgium, the Netherlands, and France, ending the Phony War and beginning the Battle of France. The British Expeditionary Force (BEF) and the French Army advanced into northern Belgium, planning on fighting a mobile war in the north while maintaing a static continuous front along the Maginot Line further south. The Allied plans were immediately smashed by the most classic example in history of Blitzkrieg.

In the first phase of the invasion, Fall Gelb (Case Yellow), the Wehrmacht's Panzergruppe von Kleist raced through the Ardennes, broke the French line at Sedan, then slashed across northern France to the English Channel, splitting the Allies in two. Meanwhile Belgium, Luxembourg, and the Netherlands fell quickly against the attack of German Army Group B. The BEF, encircled in the north, was evacuated from Dunkirk in Operation Dynamo. German forces then continued the conquest of France with Case Red, advancing behind the Maginot Line and near the coast. France signed an armistice with Germany on 22 June 1940, leading to the establishment of the Vichy France puppet government in the unoccupied part of France.

In June 1940 the Soviet Union occupied Latvia, Lithuania, and Estonia, and annexed Bessarabia and Northern Bukovina from Romania.

Following the defeat of France, Britain chose to fight on, so Germany began preparations in summer of 1940 to invade Britain (Operation Sealion). The first step necessary was for the Luftwaffe to secure control of the air over Britain by defeating the Royal Air Force. The war between the two air forces became known as the Battle of Britain. The Luftwaffe initially targeted RAF Fighter Command but thinking the results poor the Luftwaffe later turned to terror bombing London. The Germans failed to defeat the Royal Air Force, and Operation Sea Lion was postponed and eventually cancelled.

At sea in the Battle of the Atlantic, German U-Boats continued their attempt to deprive the British Isles of necessary Lend Lease cargo from the United States. The U-Boats reduced shipments considerably; however, the United Kingdom refused to seek peace, with Prime Minister Winston Churchill vowing "We shall never surrender". President Roosevelt announced a shift in the American stance from neutrality to "non-belligerency".

The Mediterranean

The Italian declaration of war in June 1940 challenged the British supremacy of the Mediterranean, a supremacy hinged on Gibraltar, Malta, and Alexandria. While Gibraltar was never under direct attack, Alexandria and Malta were hit repeatedly by Axis attacks; the thrusts towards the Suez Canal for the former, and the 1940/42 Blitz for the latter, which made the island of Malta the most heavily bombed place on earth.

Italian troops invaded and captured British Somaliland in August 1940.

In September 1940 the North African Campaign began when Italian forces in Libya attacked British forces in Egypt. The aim was to make Egypt an Italian possession, especially the vital Suez Canal near Alexandria. British, Indian and Australian forces counter-attacked in Operation Compass, but this offensive stopped in 1941 when much of the Commonwealth forces were transferred to Greece to defend it from German attack. However, German forces (known later as the Afrika Korps) under General Erwin Rommel landed in Libya and renewed the assault on Egypt.

Italy invaded Greece on 28 October 1940 from bases in Albania. Greek forces successfully repelled the Italian attacks and launched a full-scale counter-attack deep into Albania. By mid-December the Greeks occupied one-fourth of Albania.