If it hasn't been brought up already, I would like to add that the Military Tradition civic is also very useful for the flanking and support bonuses, which can make or break an early war. Also, I haven't had much direct experience with the Diplomatic Victory. But one thing that I've noticed is that Emergencies seem to be VERY important for a Diplomatic Victory. Most (if not all) emergencies grant you a diplo victory point, and I've failed a couple attempts at Diplo Vic because I didn't get any VCs from emergencies, and there just weren't enough World Congress sessions for me to earn all 10. A strong army, and a strong religious game seem to be the two best ways to win emergencies, since most of the ones that I've seen (outside of aid requests) are to liberate a conquered capital / city state, or de-convert a converted holy city.