I've been playing as the Ottomans to learn the game and a strategy I've been attempting to pull off is to vassalise Fezzan so I can take cores on the Tunisian coast. I can then steal their coastal province to take cores on the Tunisian coastline. The aim is to eventually take over Granada as a convenient landing point to start messing up Castille, Aragon and Portugal.

I've not managed to get this to work consistently yet. What tends to keep happening is that Tunis goes to war with Fezzan and taking over which means I no longer have a convenient vassal to use to CB Tunis with. To prevent this I started guaranteeing Fezzan and allying them as an opening move but I noticed that Fezzan will never call me into the war.

Any idea why? Could it be because I'm fighting my own war?