World Government

Researching World Government makes all your battle victories instant. Capture a city and you will assimilate it as soon as your infantry takes it. You can then immediately take over trade routes, resource gathering, repair, and everything else active in the city.

Capitol victories are also instantaneous after researching World Government - take an opposing capitol and you take the entire opposing territory right away.

After capitol victories, units from the defeated army remain on the field to make life difficult for you. In fact, there's no guarantee you'll be able to hold the opposing territory if your opponent can retake cities with existing units and recover some economic and military power.