The thread is very simple, near to be stupid but I consider it very interesting.
The idea is that depending of army/troop origin (culture or inclusive religion), and the kingdom or military commander who are serving, the troops maybe, fighting with another state decide to support the opponents. For example to give a better explain. If you are playing as a Ptolemaic and you are fighting any other faction, your egyptian (native) troops maybe decide to join the enemy side, during the war or suddenly in a battle. Or another option, for example as a Roman, if you have troops from samnite culture origin, and you are fighting against a samnitic city, they very probably decide to join your enemy army.
The idea is that depending of army/troop origin (culture or inclusive religion), and the kingdom or military commander who are serving, the troops maybe, fighting with another state decide to support the opponents. For example to give a better explain. If you are playing as a Ptolemaic and you are fighting any other faction, your egyptian (native) troops maybe decide to join the enemy side, during the war or suddenly in a battle. Or another option, for example as a Roman, if you have troops from samnite culture origin, and you are fighting against a samnitic city, they very probably decide to join your enemy army.