While playing the grand campaign for the first time as Germany (Kronprinz and Conrad plans chosen), Greece joined the central powers on turn 5. Yay, I guess? Sadly, the message didn't say why, which is kind of important information to have. Turns out the French declared war on them (I peeked at the Western Entente's messages). While I could create a plausible story in my head for this action (France demands access to a supply route to Serbia, Greece refuses to compromise its sovereignty or neutrality, France declares war), the repercussions surprised me.
For France's act of aggression on Greece, they were rewarded with:
+10 alignment to Turkey
+10 alignment to Italy
+10 alignment to Bulgaria
+5 alignment to Romania
Turkey, Italy, and Bulgaria moving toward the Entente makes some sense as all 3 had animosity and territorial designs on Greece and you can't grab the land of an ally. If i had a quibble it is the size of the swing. 10 points is 5 lucky unopposed rolls. The bonus to Romania baffles me, though. I would like to know the designers thoughts on this choice.
Looking at the broader world, did France get dinged with a minus to any of the neutrals outside of the Mediterranean? Survey says no. No effect on America, GB, Scandinavia, or the Middle East.
So, in conclusion, it would seem that declaring war on Greece is a great idea for the Western Entente. The entire Balkans will applaud your aggression and the wider world will ignore it.