I remember a vanilla game where the byzantines with whom i had maxed relations, decided to invade me. Sillies.

But that was an odd ball. In general, it depends on your reputation, i think. In that same game, i had a reliable reputation, and my allies were a reflection of that. Then i started exploring with the agents (one of my first games) and many assassins got caught, giving me a treacherous (don remember the exact name). I nearly had a dogpile at that point. Lost respect.

In any case, diplomatic relations can be modded. I remember a mod, i think it was vader's, where diplomacy was insane... if you had maxed out repwith someone, you will not get betrayed. And allies have maxed out, so they dont betray you... ever. And, they dont refuse an alliance out of principle... so at the start you can ally the whole bunch o' them. And... since they like you so much they dont wanna attack your allies either... i remember my first game with that mod was a Total Peace game... booooring.