With the outbreak of the Second World War the army was once more deployed to France. In 1940, 3 RTR, commanded by Lieutenant Colonel Reginald Keller, was preparing for deployment as part of 1st Armoured Division when it was diverted at short notice to Calais. Here it fought during the four-day Siege of Calais, part of the Battle of France. All of its tanks were lost, and many personnel were killed or taken prisoner, but some escaped to Dunkirk or were evacuated from Calais before the port fell.