It is after WW1
WW1 was hard and impactful. A lot has changed. The Ottoman Empire has fallen. Not only that, but the lovers known as the German Empire and Austria Hungary, and the main leaders of the Central Powers, has been killed. The German Empire is now proceeded by one of his sons, Weimar. After years and years of being mortal enemies, France and Britain, have actually learned to get along due to pairing up in this war on the Allies side of the war. While they still bicker, it's more in good fun. The Russian Empire, in the Allied powers, has also been killed, and is being preceded by his son, the Soviet Union. Now Russia is not just ruled by a tsar, but under the rule of a communist party.
Again, so many empires have fallen and nations had changed. If it's for the good or better? That's up to debate. But what's for sure is that the Central Powers had lost the war, and the Allied Powers had gained a lot for it. Especially Britain and France, who took the German Empire's territories after they killed him with the war reparations that destroyed him and his empire. As well as the Japanese Empire, who is thriving on the gains that comes with being on the winning side of a war, such a territorial gains in China.
It is now new year's eve, December 31st of 1918. The allies, at least what's left of them, are gathered to celebrate the new years, and reflect on... Everything that's happened these past 365 days.