I thought you had to use your politic action to cancel a pack, then on next turn you could aggression/war (i play with 2 players so we don't do pact, so I am not sure).
It depends on the pact. There are some pacts that say that you cannot declare a war/aggression, and there are others that say they are cancelled if you do.
A pact of the former type needs to be cancelled in one turn (either directly or by initiating another pact), and then a war or aggression the next turn. A pact of the second type cancels as soon as the war/aggression is declared.