What happens on the games I've finished as the UK and other countries, sometime immediately after WWII finishes around '43-44 the Soviet bloc, strengthened by puppeting China in a the combined peace deal, declares war on the Allies. I presume you know how factions grow during war, especially with German focuses as everyone is drawn in and joins the Allies, so what tends to happen is my Allies smashes the USSR in a war around '45-46.
If you're playing as Greece, and so unlikely to be in the firing line, it's up to you. I like indulging in the Air game as my friends can handle the land by this point but it's up to you and how involved you are.
The most annoying thing that happens is the USA suddenly completing a focus to attack Germany, which has been puppeted and joined the Allies after WWII. Then things get really interesting if you accept a disarmed Germany's call to arms against the whole Allies faction.