In 1916, he was commander of the IV Army Corps in eastern Macedonia, with his headquarters at Kavala. The corps had been demobilized by the royal Greek government in Athens and numbered a fraction of its full strength. When the Bulgarian Army, with some German units, invaded eastern Macedonia in August 1916, Hatzopoulos was forbidden by the Greek government to offer any resistance. As a result, he and the bulk of his men—464 officers and 6373 soldiers—surrendered to the Germans and were interned for the rest of the war at Görlitz. These events provoked immediate reaction among the Greek officer corps, with the outbreak of a French-backed military revolt in Thessaloniki and the establishment of the Provisional Government of National Defence there, eventually leading to Greece's formal entry into World War I on the side of the Entente.