Greece has for some time now been divided between the followers of Venizelos, the pro-Allied politician, and King Constantine, who wishes to keep the country out of the war. Allied forces have established themselves in the northern port of Salonika and are trying to contest the Central Powers’ control of the Balkans. Venizelos established a pro-Allied government there too but Constantine remained ensconced in Athens.
A previous French attempt to oust Constantine failed. Now the French have another go, occupying key points in southern Greece and presenting an ultimatum to Constantine, demanding his abdication. This time Constantine accepts defeat, agreeing to leave Greece for exile in Switzerland (but not formally renouncing his throne).
Constantine’s second son Alexander is installed as king, bypassing his elder brother George who is believed to have pro-German sympathies. Alexander agrees to recall Venizelos to the capital and invite him to form a government. With many of Constantine’s supporters following him into exile, the way is now set for Greece to formally join the Allies.
King Constantine (Encyclopaedia Britannica)
Eleftherios Venizelos (Wikipedia)