At dawn on 1 September 1939 German forces crossed into Poland. Britain and France declared war on 3 September, honoring their guarantee, but could do little to help. Polish forces fought hard, yet on 17 September the Soviet Union invaded Poland from the east, as the secret protocol allowed. Warsaw surrendered at the end of the month and the country was carved up. Both occupiers set out to destroy Poland's educated class. The Germans shot thousands of teachers, priests and officials, and in the spring of 1940 the Soviet secret police murdered some 22,000 Polish officers and others in the Katyn massacre, a crime Moscow blamed on Germany for nearly fifty years.

The Phony War and the north

In the west, months passed with little fighting, a lull the British called the Phony War. In the east, the Soviet Union attacked Finland in November 1939; the Finns held out for three and a half months before giving up territory in the Winter War. In April 1940 Germany seized Denmark and invaded Norway to secure iron ore shipments and naval bases. The failed Allied campaign in Norway brought down Chamberlain, and Winston Churchill became prime minister on 10 May 1940.

The fall of France

That same day Germany attacked the Netherlands, Belgium and Luxembourg. German bombers struck the center of Rotterdam on 14 May, killing several hundred civilians, and the Dutch surrendered the next day. The Allies rushed north to meet them, just as the German plan intended. The main blow fell further south: German tanks pushed through the wooded Ardennes, which French planners had judged too difficult for large armored forces, crossed the Meuse at Sedan and raced to the Channel coast, trapping the Allied armies in Belgium. Between 26 May and 4 June about 338,000 British and French troops were evacuated by sea from Dunkirk on warships and a flotilla of civilian boats, though they left their heavy equipment behind. Paris fell on 14 June, and on 22 June France signed an armistice. Fearing the French fleet would fall into German hands, Britain attacked it at Mers-el-Kebir in Algeria in July, killing nearly 1,300 French sailors, a grim sign of how far it would go to survive. Germany occupied the north and the Atlantic coast, while a regime led by Marshal Philippe Petain, based in the spa town of Vichy, governed the rest and chose collaboration with Germany.

The swift victory gave rise to the word blitzkrieg, lightning war, and to an image of a machine-age army. Historians have since corrected the picture. Most of the German army still moved on foot and relied on horses to pull its supplies and guns. The breakthrough came from concentrating tanks and aircraft at one point, from bold local commanders and from Allied mistakes, more than from a master plan for a new kind of war.

Britain alone

Italy joined the war in June 1940. Hitler expected Britain to make peace, but Churchill's government refused. In speeches that spring and summer Churchill promised his people only hard struggle and vowed to fight on whatever came; his words did not win battles, but they helped persuade Britons and Americans that Britain would not quit. An invasion needed control of the skies over the Channel, so from July to October the Luftwaffe fought the Royal Air Force in the Battle of Britain. Radar stations and a centralized system for directing fighters helped the defenders, and Germany failed to win the air superiority an invasion required. The Luftwaffe turned to bombing cities at night. This Blitz killed over 40,000 British civilians by May 1941. At sea, German submarines hunted the convoys that kept Britain fed, the opening of the long Battle of the Atlantic.

Fighting spread south. Italian offensives in North Africa and Greece went badly, and Germany came to Mussolini's aid, sending troops to Libya and, in April 1941, overrunning Yugoslavia and Greece. Across the Atlantic, the United States was still neutral but no longer impartial. In March 1941 Congress passed Lend-Lease, which let the President supply weapons to Britain and to other countries whose defense mattered to American security, with payment deferred. In August Roosevelt and Churchill issued the Atlantic Charter, setting out shared aims including self-determination.

Worked example: reading a map of June 1941

Look at Europe on the eve of the German attack on the Soviet Union. Germany directly or indirectly controls France, the Low Countries, Denmark, Norway, Poland, the Czech lands, Austria, Yugoslavia and Greece; Italy, Hungary, Romania, Slovakia and Bulgaria are its allies. Britain holds out with its empire and American supplies. A student might conclude that Germany had already won. But the map hides weaknesses. Much of Germany's oil came from Romania, its food stocks were tight, and Britain's survival meant a blockade and a base for future attacks. Each German victory had widened the war rather than ended it. This is a useful habit in any history: ask what a moment of apparent triumph cost, and who is still fighting.