At the outbreak of the
Second World War Stewart was still in command of the 2nd Battalion of the Argylls. Stewart was one of the few British officers to realise the need for training in jungle warfare that would be necessary in order to defeat the Japanese in a war in Malaya. When the 2nd Argylls were thrown into the battle in early December 1941, the battalion was one of the few effective units the Japanese would face in their rapid advance down the peninsula, inflicting heavy casualties in every engagement. The Argylls' effectiveness meant that the battalion was continuously used as the buffer and suffered heavy casualties as a result. When the
Battle of Malaya finally ended and the surviving Allied soldiers retreated across the causeway onto Singapore Island, Stewart and his
batman, Drummer Hardy, were the last to cross. The Plymouth Argylls suffered heavy casualties during the brief
Battle of Singapore. Stewart was evacuated unwillingly from Singapore before its surrender due to the need for experienced officers and men who had proven ability to fight the Japanese Army, an ability rare in the British Army at this time. By the time of the surrender on 15 February, the Plymouth Argylls were reduced to 40 officers and men. He was initially employed as a General Staff Officer Grade 1 in India, lecturing on his experiences in Malaya. Officers with jungle warfare experience who had actually fought the Japanese and seen their tactics in action were in high demand by the army command in India. Of the officers to have escaped from Singapore, Stewart was one of the most experienced. He became the Chief Instructor at the School of Infantry as a
colonel, and later Brigadier General Staff (Training) with 11th Army Group in India. Of Stewart, Field Marshal
Archibald Wavell wrote: Stewart's report, written after his arrival in India, and his knowledge along with that of the other officers and men who escaped from Singapore, had a direct effect on the training and tactics that would be used by the British and Commonwealth Armies in fighting the Japanese throughout the rest of the war. He spent a period of time as the District Commander for Stirling, the home of the Argyll and Sutherland Highlanders. ==Retirement==