Quote Originally Posted by PseRamesses
Changing sides? I´ve never encountered that in M2. As a cath I´m allied with all other cath´s and they are at war to the left and right but I´ve never been forced to end an alliance with either of them. Occasionally the pope ask you to terminate an alliance with some x-commed faction but that is another matter.
I did not pay attention to this aspect in MTW2 vanilla (before any patches). If my memory does not fail me, the player was not forced to choose sides when allies attacked each other. This is no longer true in 1.02. If one of the player's allies attacks another, there is a pop-up that comes up asking whether the player would like to stay allied to the attacker. If the answer is "no" (or, to be more precise, "cancel"), the player remains allied with the defender, losing his alliance to the attacker. If the answer is "yes", the player remains allied to the attacker and loses alliance to the defender.

Player receives reputation hit in both cases (remaining allied to the attacker or remaining allied to the defender).