Slitherine

As a result of the Balkan Wars Greece increased the extent of its territory and population. In the following years, the struggle between King Constantine I and charismatic Prime Minister Eleftherios Venizelos over the country's foreign policy on the eve of World War I dominated the country's political scene, and divided the country into two opposing groups. During part of WWI, Greece had two governments; a royalist pro-German government in Athens and a Venizelist pro-Britain one in Thessaloniki. The two governments were united in 1917, when Greece officially entered the war on the side of the Triple Entente.

http://en.wikipedia.org/wiki/Greece

But in the game Greece does not enter Entente in 1917. Why?