Following the completion of the fighting in Sicily, Durnford-Slater took over operational command of both No. 3 and No. 40 (Royal Marine) Commandos along with the Special Raiding Squadron, operating them as a brigade. In August and September 1943 he commanded them during Operation Baytown, as the Allies commenced their invasion of Italy. In early October he led the detachment in Operation Devon, where they were employed to spearhead the assault on Termoli, on the east coast of Italy, about 120 miles north of Bari. In the end the port was captured largely as a result of Durnford-Slater's commandos. The town was taken despite intelligence failures, it was expected that they would face only administrative units of the German 1st Parachute Division, but in fact 26th Panzer Division were in the area, being rested following the fighting at Salerno; and problems during the landing, some landing craft of 40 Commando and the SRS grounding fifty yards offshore in six feet of water, leading to the loss of all their radios as the men waded ashore. Their success meant 8th Army was able to move forward instead of fighting for the line of the Biferno River, whose mouth was at Termoli. The capture of port closer to the front line also enabled the Allies to shorten their supply lines, and opened a road from Naples to the east coast.