The early years of the Second World War had seen Germany progress from one astounding triumph to another. In September 1939, Poland was crushed within weeks, and, in May 1940, Germany succeeded beyond its wildest dreams by conquering Holland, Belgium and France in a similarly rapid fashion. Britain stood alone in defiance of the Nazi onslaught, its Army having narrowly avoided annihilation by withdrawing from the Dunkirk beaches.
Britain escaped the threat of invasion when the Royal Air Force emerged victorious in the Battle of Britain, however Germany remained supreme and unassailable. In late 1940, Italy turned its attention on British interests in North Africa, but was heavily defeated by the comparatively small force based there. Germany came to the rescue of their Italian allies and, after a year of fighting in which both sides had flirted with victory and defeat, the tide swung against the British in June 1942, when Erwin Rommel's Afrika Korps pushed the 8th Army deep into Egypt, threatening the oil fields of the Middle East and the Suez Canal, the lifeline connecting Britain with its Empire.
Adolf Hitler did not consider Britain to be a threat at this time, and so turned his attention towards Russia. In June 1941, his armies attacked and swept all before them, accounting for Russian soldiers by the million, and within weeks an area twice the size of Germany had been captured, the front line stretching from Leningrad in the north, down to Moscow and on to Rostov in the south. Germany, however, could not achieve complete victory before the Russian winter set in, its harsh temperatures paralysed the Wehrmacht, but gave the Russians time to regroup. The German advance was resumed in the spring of 1942, this time in the south, taking Rostov and pushing on to Stalingrad. Hitler's obsession with the capture of this city resulted in a defeat from which Germany would not recover. The bitter fighting through the streets of Stalingrad cost both sides approximately one million men, losses that Germany could ill-afford. In November 1942, the Russians counterattacked around the flanks of Stalingrad and succeeded in trapping two hundred and fifty thousand German soldiers, only six thousand of whom survived the War.
A month before the victory at Stalingrad, the British in North Africa had also achieved their first great success of the War when the 8th Army, under General Montgomery, won decisevely at the Second Battle of El Alamein and proceeded to drive the Afrika Korps back to Tunisia. The United States of America had entered the war in December 1941, and by June 1943, they had helped the 8th Army to push the Germans and Italians out of North Africa for good.
The summer of 1943 was a particularly dark time for Germany. Defeat in North Africa was followed by a British and American capture of Sicily which soon led to an invasion of the Italian mainland, prompting their immediate surrender. In Russia, this success was matched by victory at Kursk, where Germany had gambled its last offensive strength on an ambitious counterattack which failed, following which the Red Army mounted their own counterattack and pushed the Wehrmacht ever backward. The days of unbounded German triumph were gone, and they did not return.
To add to their woes, the Battle of the Atlantic, a four year long struggle between the German U-Boats and British merchant shipping, resulted in victory for the Allies, allowing scores of troops and equipment to cross safely to Britain from the USA. By 1944, Britain more resembled a vast military base than it did a country. The once mighty Luftwaffe had now been so reduced in strength that the Allies enjoyed considerable air superiority over the skies of Europe, and through the sustained bombing of industrial and military targets, a tight grip was maintained around the throat of German resources.
Pitted against such powerful opponents, with insurmountable quantities of men and war materiel at their disposal, the writing was on the wall for Germany. But in spite of their dire position, they were still very much in complete control of the Europe that they had conquered, and their armies remained a potent and unbroken force. To finally defeat Germany, and relieve the pressure off the Russians, the British and Americans had to invade Europe.