The squadron's involvement in the fighting came on 8 December 1941, following a heavy Japanese bombing raid on Sungai Petani, which resulted in the loss or damage of several of its aircraft. Initial interception operations proved unfruitful and, as a result of heavy damage to the airfield, the squadron was withdrawn to Ipoh, falling back through Butterworth, where they squadron achieved its first air-to-air victory, when a Buffalo piloted by Flying Officer Harold Montefiore shot down a Mitsubishi A6M Zero single-engined fighter. Despite this success, though, No. 21 Squadron suffered severe losses to Japanese fighters, on the ground and in the air, during the first week of the campaign; replacements were only limited, resulting in the squadron's operational merger with 453 Squadron.