Military agreement
From Rival Regions Wiki
Military agreement, just as Migration agreement, can be accepted between two states unilaterally or bilaterally by passing the law.
When state A passes agreement with state B it allows any players from state A to take part in wars of state B.
In the case of both states passing this laws this agreement will be called Bilateral.
You can see the list of this agreements on the state page.
Blocs
Inside the political blocs members can vote on the in-bloc agreements.
In that case all the players from any region of the in-bloc states can take part in any in-bloc war.