He was named as the commanding officer of the newly raised 9th Infantry Division in 1913, and commanded it until 1916, when he joined the Venizelist Movement of National Defence in Thessaloniki, which, in opposition to the royal government in Athens, entered World War I on the side of the Entente Powers. Ioannou was tasked with the formation of the Archipelago Division, drawn from the inhabitants of the Aegean islands. By May 1917, his division was ready and deployed in the Monastir sector of the Macedonian front. He led his division to victory at the Battle of Skra-di-Legen in May 1918, and participated with it in the general Allied offensive of September 1918, which broke the German-Bulgarian front. The armistice with Bulgaria found him with his division at Pehčevo.