For some time now Greece has been divided between a pro-Allied government headed by Eleftherios Venizelos based in Salonika and a neutralist government in Athens loyal to King Constantine. But now the Allies have successfully forced the Constantine into exile. Venizelos returns to Athens and establishes a pro-Allied government there whose writ will run over all of Greece, albeit relying on Allied arms to intimidate any remaining supporters of Constantine.
Today Venizelos finally achieves his great ambition: declaring war against the Central Powers and bringing Greece into the war on the side of the Allies. The Allies hope this will allow them to increase the pressure on the Central Powers in the southern Balkans.
A pro-war propaganda poster (Wikipedia)