Diplomacy | Medieval Kingdoms Wiki

Press T on the map. Every offer is made there, and the other nation's leader accepts or refuses it there.

Alliance: either side can call the other into its wars, and the ally chooses whether to answer.
Defensive Pact: attack one and you have attacked both. The partner joins on the defending side automatically. It only works in defence: a nation that starts a war cannot drag its partner in.
Non-Aggression: a promise not to attack each other.
Trade Agreement: each side receives a quarter of the other's surplus goods every turn.
Military Access: each side's armies may march through the other's provinces. Passing through takes nothing: only a war lets you take land. Militia still stay home.
Truce: starts by itself after every peace and lasts 4 turns. No war can be declared between the two while it lasts.

Any pact except a truce can be broken. It is legal, but everyone sees it, and every province of the nation that broke it gains unrest.
Declaring war on a nation ends every pact you held with it, apart from a truce, which stops the declaration.