On 1 September, 1939, German forces stormed into Poland, precipitating nearly six years of savage, unrelenting warfare that would leave large areas of the world in ruins. For the first three years of the conflict, German forces seemed invincible. The swift victory in Poland was followed by the rapid conquest of Denmark and Norway, bringing the Phoney War to an abrupt end. The seeming invincibility of German arms was further strengthened when, in May 1940, the Germans launched their Blitzkrieg in the west, overwhelming Holland and Belgium in a matter of days, driving the British Expeditionary Force back across the Channel and completing their campaign in France within a month. The victory of the Royal Air Force over the Luftwaffe in the high summer of 1940 proved that the Germans could be beaten, and the Royal Navy's early victories in the Mediterranean showed the weakness of Germany's Axis partner, Italy, whose performance in the Balkans and North Africa was little short of disastrous. The Italians' lack of performance was one of the factors that compelled the Germans to invade Yugoslavia and Greece, in order to secure their southern flank before launching an invasion of the Soviet Union in June 1941. This was to prove the biggest mistake of all, but in the short term the German forces swept from one victory to another, their armoured divisions storming across the vast Russian plains to within sight of Moscow. In December 1941 the United States entered the war, but Brit