Greece, with its strategic position and well-known enmity towards the Turks, was a sought-after ally of the French and British. In 1915, Winston Churchill (then First Lord of the Admiralty) suggested they might like to undertake the attack on the Dardanelles on behalf of the allies. However, the Greeks were unwilling to commit to the allies, probably largely due to the Pro-German sympathies of King Constantine, married to the Kaiser's sister.
Later on that year, the British and French again attempted to woo the Greeks with promises of territorial gain in Turkey. Still, the Greeks didn't commit themselves to war. However, they did allow British and French forces to disembark at Salonika en route to Serbia. By the time the allied forces were ready, Serbia had already fallen and Austrian and Bulgarian troops stood on the Greek border. Trying to avoid being sucked into the war, Greece took an even-handed approach. She promised to "neither help nor hinder" the allied armies, whilst at the same time handing over the key fortress of Rupel to the Bulgarians in the summer of 1916.
The Austrian and Bulgarian attack on Greece was staved off by the British and French forces until Constantine abdicated under French pressure on the 12th June 1917 and his son declared war on the Central Powers on the 2nd July. Greek soldiers fought alongside the French and British troops in Greece, then pushing back into Serbia.
About 5,000 Greek soldiers died in the 18 months she was at war.
King Constantine of Greece