Eighty years ago next month, on 22 June 1941, Adolf Hitler unleashed the largest military invasion force in history, when he ordered almost 3,700,000 Axis troops with 3,000 tanks, 7,000 guns and 2,300 aircraft to advance into the Soviet Union along a front that extended from the Baltic to the Black Sea. Less than six months later, following startling early successes, leading German units would reach almost to the gates of Moscow, before plummeting temperatures and the onset of the Russian winter finally brought their advance to a halt.