After World War II began, Cariappa was posted as Brigade Major to the 20th Indian Brigade stationed in Derajat. Later he was appointed as the DAQMG of 10th Indian Division which was stationed in Iraq. He earned a Mention in Despatches as DAA and quartermaster general of General (later Field Marshal) William Slim's 10th Division. He served in Iraq, Iran and Syria in 1941–1942 and then in Burma in 1943–1944. Back in India in March 1942, he was posted as second-in-command of the newly raised 7th Rajput Machine Gun Battalion at Fatehgarh. On 15 April 1942, he was promoted to acting lieutenant colonel and was appointed commanding officer of the same battalion, receiving an advancement to temporary lieutenant-colonel on 15 July. With this appointment he became the first Indian to command a battalion in the Indian Army. Cariappa was successful in stabilising the newly raised battalion in terms of administration, training, and the handling of arms. Later the unit was rechristened as the 52nd Rajput and put under 43rd Indian Armoured Division. Within a span of a few months, the unit observed two transformations and two moves. First, the battalion's machine guns were replaced with tanks in order to convert it into an armoured regiment. But soon the battalion was reverted to infantry and re-designated as 17/7 Rajputs. Subsequently, it was moved to Secunderabad. This move led to unrest among the unit's troops which was successfully handled by Cariappa.