I dipped back into Civilization 5 again recently. For the first time in a playthrough I asked another civ to go to war with me against another and they actually said "yup let's do it."
We crushed Genghis Khan together. I took his capital, liberated the city states for the alliances/negating warmongering penalties, and left him with a single landlocked city. I warned you not to touch Sydney, you butt.