Matthews' active service started in July 1943, landing in Sicily and directing the naval gunfire of HMS "Roberts" and later finding artillery locations, while under direct enemy fire, for the assault on Agira during the Sicily campaign. In September 1943, his Italian mainland campaign began: he was later awarded the DSO. He returned to the UK in January 1944, having been transferred to the II Canadian Corps as CCRA, regarded as the "second most senior job for a gunner in the Canadian Forces". II Corps was activated in Normandy on July 11, 1944 and participated in several actions (Operations "Atlantic", "Spring", "Totalize" and "Tractable") in the Battle of Normandy. Matthews developed the artillery firing plans for each of these operations.