Allied armies had also reached the pre-war German territorial borders and now occupied some German towns and cities. In the West the defeat in Normandy and the Allied advance across Western Europe had significant consequences for the "Luftwaffe's" ability to defend Germany from British night attacks. The Kammhuber Line—German air defence system—which had extended through occupied France, Belgium and the Netherlands was now broken and much of its early warning network had been lost. Along with the battlefield reverses was the inability of the "Luftwaffe" to produce enough experienced night fighter crews which was exacerbated by the crippling shortage of fuel at this stage in the war which contributed to the collapse of training programs and grounded combat units. Equally serious was the threat posed by RAF de Havilland Mosquito night fighter intruders operating over Germany.