Calis
on time
Well, a really old topic, but it's rep related...
This might get you a rep hit

Situation: I've made military alliances (MA) with several civs against the Arabs. I don't share a border with Arabs and I'd like to achieve it. This would mean invading Zulus, one of my MA partners (I'm giving gpt to Zulus for the deal). If I do this, would it give me a rep hit?

And to take the question a bit further, do I get a rep hit in the following cases (assuming no gpt deals)?
1) MA with Zulus is under 20 turns old

2) MA at least 20 turns old
Make sure the MA has ended! Then you won't get a rep hit.
And finally, if I invade Zulus, are they more likely to make peace with Arabs and, furthermore, to ally with Arabs against me?
Not sure about the likelyhood, but if possible the AI will try to get other AIs into the fight.