Probable reasons why certain alliances or client states treaties aren't sustainable in the long term

[twgamer20197]

I think I got to figure out why alliances or client state treaties tend to break in the long term in a campaign:
- One reason could be that the different factions with whom you have alliances or client state treaties hate one another. That means we should be very picky when choosing factions with whom we want to keep an alliance or client state treaty for long term.
- Another reason that could instantaneously break an alliance or client state treaty with a faction is forgetting about unticking the box "Call allies for help" when you declare war to a faction or when a faction declares you war. You should always untick that box. In case you want one or more of your allies or client states to help you in a war, you'd better invite them one by one from the diplomacy section in order not to risk the alliances.
Even by doing all of that there is no guarantee that you'll keep client stares or allies during the whole campaign or most of the campaign. For example, when you receive a notification that another faction is attacking one of your allies, you'll have to make a tough decision: joining its war or not.

EDIT:
I forgot about an extra reason:
It has to do with a faction's leader characteristics. If he's unreliable or treacherous, it implies that you shouldn't establish any alliance nor client state treaty with that faction.

[Alwyn]

Yes, those make sense, and I agree that there's no guarantee that the player will keep client states or allies throughout.

In my current campaign as Baktria, my long-term ally (Arachosia) just attacked one of my satrapies, Saba. Almost all of my armies were far to the west when Arachosia attacked. Maybe their decision to attack was partly because they saw an opportunity to take some rich, lightly guarded provinces, and partly because I was reaching a level of Imperium where the player's faction gets a significant diplomatic penalty for territorial expansion. By the late campaign, there's usually at least one major war where an ally or client state attacks - I don't mind, because the ally or client state was still useful in the early campaign, and having to improvise and fight an unexpected war can be a challenge.

[twgamer20197]

Yes, those make sense, and I agree that there's no guarantee that the player will keep client states or allies throughout.

In my current campaign as Baktria, my long-term ally (Arachosia) just attacked one of my satrapies, Saba. Almost all of my armies were far to the west when Arachosia attacked. Maybe their decision to attack was partly because they saw an opportunity to take some rich, lightly guarded provinces, and partly because I was reaching a level of Imperium where the player's faction gets a significant diplomatic penalty for territorial expansion. By the late campaign, there's usually at least one major war where an ally or client state attacks - I don't mind, because the ally or client state was still useful in the early campaign, and having to improvise and fight an unexpected war can be a challenge.

The mistake of not unticking the box "Call your allies for help" was something I learned the hard way.
Another additional measures you can take in order to try to keep alliances and client states is by regularly sending diplomats to those factions. And giving them money away from time to time as well.
Maybe you should join all wars where your allies or client states/satrapies are involved each time you receive the notification that another faction is attacking an ally or client state/satrapy.

[Alwyn]

Yes, these additional measures help. I've noticed that factions appreciate it when you send diplomats to their allies (and dislike the player's faction when you send diplomats to their enemies), too.

[twgamer20197]

Then it means you should make treaties only with factions that like one another

[JaM]

There are multiple other things deciding these, like reliability and aggresivenes of particular ally.. check it on diplomacy screen.. those who are unreliable will stab you more often.. loyal ones wont betray you