Two main elements hampered Germany's effort: While pure Soviet manpower resulted in disastrous defeat in Russia, US involvement increased the allies' arms-producing capacity. In September 1941, two years into the war, German arms seemed to be all before them. Western Europe had been clearly subjugated, and there was little evidence of any major opposition to German control. German intervention had made good the Italians' inability to create Mussolini's much-vaunted new Roman empire in the Mediterranean.
German troops had conquered Yugoslavia and overrun Greece. Rommel's superb generalship was threatening the Suez canal and pushing the British and allied forces eastwards towards Egypt in north Africa. Above all, the invasion of the Soviet Union in June 1941 had paid off handsomely; Leningrad (the modern-day St Petersburg) was besieged by German and Finnish troops, Smolensk and Kiev taken, and millions of Red Army troops killed or captured in a sequence of large-scale encircling operations brought the German armed forces within Moscow reach. Surrounded by a girdle of supporters from Vichy France and Finland to Romania and Hungary, and with the almost benign neutrality of nations like Sweden and Switzerland posing no major threat, the Greater German Reich seemed to be unstoppable in its drive for supremacy in Europe.
Looking back, though, this proved to be the pinnacle of German achievement. The basic issue confronting Hitler was Germany's lack of means to fight on so many several fronts simultaneously. Already starting to see this were top economic managers like Fritz Todt. The young Albert Speer replaced Todt as armaments minister when he was killed in an aircraft collision on February 8, 1942. Driven by an unquestioning faith in Hitler and his will to win, Speer rebuilt and rationalised the arms production system, building on Todt's already started reforms. His techniques improved the supply of ammunition to the troops and helped greatly increase the quantity of planes and tanks produced in German plants.