He was subsequently appointed to the Army Staff Service, and served in it until 1913, being promoted to captain (1902), major (1910) and lieutenant colonel (1912).

Due to the mobilization ordered in September 1915, when Greek PM Eleftherios Venizelos prepared to come to Serbia's aid against the Austrian attack, the Academy was dismissed and Vlachopoulos was placed in command of the V Army Corps at Patras.

After King Constantine's forced exile in June and the assumption of leadership by Venizelos, Vlachopoulos was suspended from active service and forcibly retired.

[2] Following the Venizelist electoral defeat in November 1920, he was recalled to active duty and placed as Chief of the Army Staff Service.

He was then placed in charge of the Asia Minor Superior General Military Command, which covered the rear areas of the Army of Asia Minor, and in June 1922 he was dispatched to Eastern Thrace to assume command of the IV Army Corps.