Bringing a civ back from the dead by liberating them - from then on, they become near-permanent allies in gratitude.
When you have "Complete Kills" on (requiring every unit as well as city to be destroyed before a Civilization is gone) and take every city of a civ, leaving just a few crippled units as all that's left of it in the world.
The simplicity of "One extra happy citizen in every city" manages to evoke a strong emotional response.