There are two types of treaties, common and private ones.
Private treaties can be proposed and signed between two players at any time as long as there is no common treaty binding them.
In order to sign a private treaty, you have to open the player's personal information screen and send a treaty proposal.
Once sent, the other player will find this proposal in the message box - if accepted, the treaty will be signed and effective immediately, replacing any former treaties between the two parties.
Whenever a proposal is being denied, the current treaty - if existent - will remain valid.
Private treaties can be cancelled at any time on your personal information screen.
Every signed treaty affects only military issues, conflicts which are criminal in nature won't be affected.
Treaties also apply to assassinations, rebellions and holy wars.

You can choose from the following list of treaties:
DescriptionAttack clauseDefense clauseAttackprohibition
Alliance treatyyesyesyes
Anti-aggression treatynoyesyes
Non-aggression treatynonoyes
Fragile Alliance treatyyesyesno
The attack clause obligates both parties to support each other when attacking an enemy.
The defense clause obligates both parties to support each other when being attacked by an enemy.
Treaties that prohibit direct engagements in battle obligate both parties never to attack each other.
Whenever players are bound to assist both parties in a conflict, they will remain neutral.

In every conflict that occurs between two players, only those treaties signed directly with either of both players count.
Even though players might have a signed treaty that prohibits a battle between them, they still could engage each other in battle.
However, this can only happen, when both parties intervene in a conflict bound to their treaties.

Common treaties are being signed between unions and their members by their founders.
They can only be cancelled after leaving a union or by their founders.