When a civilization is removed from the game in Sid Meier’s Civilization, typically the civilization’s cities are taken over by the invader with lots of pillaged tiles and population lost. This also typically results in reduced happiness and amenities in the conquering civilization’s existing cities and in rare cases may lead to an armed rebellion or secession if happiness is already too low. But it does increase the conquering nation’s overall score and eventually in 20 turns or so the conquered lands will be fully integrated