By the spring of 1917, it comprised three infantry divisions that formed the National Defence Army Corps (Σώμα Στρατού Εθνικής Αμύνης) and fought in the Macedonian front.

The question of Greece's participation in World War I had led to acute political divisions, with the pro-Allied Prime Minister Eleftherios Venizelos opposed by the pro-German King Constantine I, who favoured neutrality.

On 5 August 1916[1], the Bulgarian invasion of eastern Macedonia commenced, facing little resistance, since the Athens government refused to condone any firm action.

The surrender of the hard-won territories gained in the recent Balkan Wars led to a military mutiny in Thessaloniki on 17 August.

On 2 September, the "National Defence" received its first substantial reinforcement, as Colonel Nikolaos Christodoulou arrived in the city with the remnants of units of the IV Army Corps that had refused to surrender to the Bulgarians and instead withdrawn via Kavala and Thasos.

However, it controlled Macedonia, Crete and the Aegean islands, and hoped that it would attract volunteers from southern Greece—following pressure by the Allies, the Athens government gave its consent to allow this, provided that they resigned any office first—allowing it to create an army of 80,000–90,000 men.

[8] The Thessaloniki and Cyclades Divisions existed only at the depot level, and eventually were never established due to the lack in equipment and personnel (particularly trained officers and NCOs).