There are currently four factions in-game. These are The United Kingdom, l'Empire Francais (French Empire), Königreich Preußen (Prussia), and Rossiyskaya imperiya (Russian Empire).
Choosing a Faction
When you join a game, you select a faction from a GUI. Server admins can set whichever team to fight against whichever team, so not all of the matches will be historically accurate. The balance threshold is currently 15 players.