Williams took part in the fighting from Normandy in 1944 and through Belgium and the Netherlands to Germany in 1945. There he was involved in liberating a smaller Nazi concentration camp, which was afterwards used by the Allies to detain SS officers. He was shocked to find that Hamburg had suffered saturation bombing by the Royal Air Force, not just military targets and docks, as they had been told. He was expecting to be sent to Burma, but as his studies had been interrupted by the war, was instead granted Class B release, which meant immediate demobilisation. He returned to Cambridge, where he found that the student culture had changed from 1941, with the left-wing involvement much diminished.