Just adding to clarifications, and please check out the FAQ and errata.
Combat on land is just one round and mandatory. However in a territory that is contested, combat is optional (for all players - even to players that just moved into that territory).
Combat at sea is optional and can be multiple rounds (active player’s choice).
Air units are free to move to any territory, unlike land units that have movement restrictions when starting in a contested territory.
The Russian Revolution procedure may not be entirely clear - please check out other posts - or feel free to ask any questions. (BTW - Big fan here too).
If Germany and France are in a contested territory and Great Britain enters that territory for the 1st time I thought they HAD TO FIGHT.?
EDIT: I reread that part in the manual and it’s clear you are right. I gotta slow down when reading