Every war needs a valid cause (see Campaign Rules).
A declaration is shown to everyone 24 hours before it takes effect, so the defender can call allies.
Defensive pact partners join the defender automatically. Allies can be called in, and any nation can join a war on either side or leave one it is in.
Taking a player's land
March an army into an enemy province. If an enemy army is there, a battle is owed (see Diplomacy Battles).
An army standing alone in an enemy province takes control of it after 12 hours. Controlling it counts toward warscore, but it is only yours to keep through a peace.
Taking unplayed land
Land no player leads has its own garrison, bigger in more developed provinces and backed by the same state's provinces next door.
An army that stops there fights the garrison at once, worked out by the map (no battle in the game). Lose, and your army falls back.
An army only marching through on its way somewhere else is let by: it neither attacks nor is attacked. Only the province it ends its march on fights.
Win, then hold the province alone for 4 days and it is yours.
24 hours after it falls, its state counterattacks once, with a full garrison plus help from its provinces next door. Your armies standing there must be stronger than that or they are pushed back and the province is lost. Clicking the province on the map shows when it comes and how strong it is against what you have there.
Beat the counterattack once and the province is yours for good.
Capitals
A capital can be occupied but can never be taken in a peace. No nation can be wiped out.
