Everyone joins in within a month at most.
Here's some of the reasoning/war aims of the participants, but this is mostly flavor, as I'm mostly interested in seeing how you guys guess this correlation of forces is going to go down, both in Europe and the colonies.
Team Orange:
Russia wants to support Serbia and take over Austrian Galicia. They desire control of the Bosphorus and Dardanelles, and will support Greek and Italian claims to Ottoman lands in return. They also want to retain their newly-gained influence over the government of Persia, and keep their lands in the far east.
Germany wants to knock down France a peg or three and absorb all German areas of Austria-Hungary, puppetizeing the rest. They also desire their own “place in the sun” by not letting Britain and France hog all the good colonies.
Italy wants it’s own chunk of Austrian lands, as well as more of the Ottoman Empire to itself. Half of Switzerland would be nice too.
Greece wants to fulfill the ‘Megali Idea’ at the expense of the Ottoman “sick man of Europe” Empire, and retake Kavala from Bulgaria. Cyprus from Britain would be nice too.
Romania wants Transilvania from A-H, and has also been promised Bessarabia from Russia in return for joining the team effort, along with heavy post-war investments.
Argentina wants the Falklands, and is convinced it will get them at the peace table once Germany wins.
Bolivia wants its acces to the sea back.
Serbia wants all southern slavic lands. Maybe even Bulgaria if the populous can be persaded…
Spain is also convinced German victory is a shoe-in. They want Gibraltar, protectorate over Morocco, German investment and, down the line, support to retake their islands from the treacherous Americans
Green Team:
France wants Alsace-Lorraine back and to maintain its colonial Empire.
Britain wants Germany to stay away from the channel and to stop trying to dominate Europe. And keep its colonies. And stop the Russians from taking the Straits. And take Germany’s colonies for themselves if possible.
Portugal has been Britain’s buddy since, like, forever, and don’t want to have their biggest trading partner upset at them.
Switzerland wants those pesky German soldiers trying to outflank the French to leave it alone.
Montenegro doesn’t want the Serbians dominating them for some reason. The King of Montenegro would also like to become King of Serbia in return for his support.
Austria-Hungary wants to survive. Would not object to getting back Silezia, and also Bavaria while they’re at it, from Germany.
Sweden doesn’t like its navy being sneak-attacked by Russia.
Belgium wants the invading German army to just go home.
Bulgaria wants its clay back from Serbia and Romania.
Ottoman Empire wants to keep what it has, and get its promised juicy low-interest loans from Britain at the end. Also an island or three from Greece, and the Dodecanese back from the treacherous Italians.
Japan wants to be acknowledged as a world power, and gobble up the German colonian empire. And the Russian far east as well.
Denmark has somehow been persuaded to join (don’t ask), and plans to block the Kiel canal and get Schleswig-Holstein at the peace table.
Chile just wants to be left alone
Wild card -
United States of America
Depending on the actions of the participants, the US might join from anywhere between 1915 or not at all. However, by authorial fiat, the US will join Team Orange by April 1917 if they still hold France and appear reasonably strong.