There is a -18 for "you occupy one of their cities" and after some checking for
@nzcamel it does not appear to degrade at all. However there is a way around it. You attack civ A and take 4 cities including the capital, you give 2 back to lower you warmongering points, keep the capital and give a city that is close to your army to your next target. You then declare a formal/joint/ casus war against civ B (surprise war warmongering points are your own stupid fault) and take the city you gave to them back and liberate it back to civ A. The -18 disaapears and instead you get a +20 for liberating one of their cities and you get +5 with every other civ for liberating a city (not stackable). Yiu then rinse and repeat the same process until every civ you have attavked but you last has +25 instead of -18.
DO NOT raze any city, this replaces the -18 with a -20 you cannot remove and does not seem to degrade.
As warmonger points degrade 1/2 turns from the moment the war starts they soon disappear so sometimes declaring war early can get rid of them early and as long as you do not raise cities and ideally loot their ground and get cities in peace deals and give back ones you took apart from the capital everyone will be your ally by the end of the game. A peaceful world with you in charge.
For the peaceful players out there, it is fine to retaliate as long as you accept that ghandi will hate you with -8 for the rest of the game (you can couteract with positives) and that you will get some short term warmongering unless you take and keep a city. You are better to raise their lands and ask for the city in a peace deal. Each mine give you +25 science, a shame in some ways they do not build more of them.