There must have been a patch with the Scrambled Nations pack.
Best patch ever.
Most of it was introduced with BNW, the patch halved the Warmongering Amount with civ also at war with the victim, and tweaked the threat values so they are more exponential than linear.
Now the warmongering hit isn't so severe for just taking one city,
It can be extremely severe depending on the city, the size of the map and when you took it. That's not important in itself, it's the effects on your diplo with other Civs you need to factor.
It's not a perfect fix... liberating a single CS shouldn't reduce the penalty quite so much, and I wish the penalty would go away with time naturally so long as you don't take another city for a good while (maybe after 30 turns reduce the diplo hit one notch).
But one of the biggest complaints going all the way back to vanilla is that the diplo hit for going to war was too severe, and was irreversible. Finally after, what, 3 years, they've made the warmongering system fairly well-rounded. Not perfect, as mentioned before, but about time.
The penalty
does reduce each turn after you take a city (5 points per turn from the raw amount, before adjustment for Warmonger hate, ie: how much a leader tolerates or not warmongering). How fast taking one city will stop having direct effects on your diplomatic relations depends on the size of the map, the total # of cities currently on it and the # of cities your victim has. In some situations, taking one city early on can have about half or less the effects of being denounced.
On a standard map in Classical, taking a city from an opponent that has 4 while there are 33 cities (on Standard the game estimates there should be 52 cities), you would get 393 as Warmonger amount. It will totally have gone away in 78 turns. The formula is (1000 * Estimated # of cities on that map) / (Present # of cities * # of cities the victim currently have). So (1000 * 52) / (33 * 4) = 393
That's not the final number used in diplomacy, as it's then adjusted for Warmonger hate (how much any leader dislike warmongering). The average is 5. Warmonger Amount * Warmonger hate / 100 is the formula. (390 * 5) / 100 = 19.
19 is the diplo modifier your warmongering gives you. Up to 20 it's a minor threat : ("They have some early concerns about your warmongering.").
What you need to be careful about is that your warmongering doesn't lead to a chain of denouncements you are not in a position to deal with... A denouncement is a diplo hit of 35. 35 from denouncement + 19 Warmongering = trouble.
If you go and stupidly take out Venice early when it doesn't have any puppet... you're in deep trouble....
(1000 * 52) / (33 * 1) = 1575 Warmongering Amount.
Adjusted for an average leader with Warmongering Hate of 5:
(1575 * 5) / 100 = 78
At 78 it's more than 2x being denounced, and on the threat level scale it stands halfway between MAJOR and SEVERE.... for an average ruler. Some leaders have a warmongering hate of 7... = 110 = SEVERE threat.
You're certain to get the chain of denouncements and to find yourself globally DoWed, and won't get out of this for the whole game as the chain of denouncements will feed itself and lead to another... unless you happen to have a CS you can liberate right before taking Venice (not
after, it's then too late to avoid the chain of denouncements), which would bring the penalty for taking Venice to zero.
The same happens if you take out a CS so early in the game on a Standard Map.
If you do take a city early, on a continent with 2 Civs and you, make sure to bring the 3rd civ in the war, as then the Warmonger Amount you get will be halved with that civ, which will probably be enough to avoid a denouncement if you have enough positive modifiers and are strong enough.
You can also perfectly take several cities early from a rival before meeting other civs if you start alone with another Civ on your continent. The other civs will never know. If you can deal with the denouncement and DoW, you can also do it if there are 2-3 civs on your continent. The rest of the world will never know... but if the other 2 Civs got caught in a loop of denouncements and become friends with other Civs later, you might still pay for that indirectly, because of the denouncements, not the Warmongering itself.
As for the "liberator" bonus, it's the mirror of Warmongering. If you liberate a CS, it's de facto always their only city, so keeping the numbers/settings from above you get -1575 Warmongering Amount for liberating a CS in the Classical Era, when there's only 33 cities on your standard map. If you liberated 1 out of 3 cities from an opponent instead, you'd get -393. This fades away at +5 points per turn.
I'm not sure, but I think the negative number you get as liberator also only applies to Civs you have met. So if you freed Lahsa from the Mongols while you knew only India, you won't stand at -1575 Warmongering Amount with anyone but India.
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So you need to be very careful which city you take and when. If you're on Pangea, you can go to war and acquire cities early, but the wise way to do it is to kill units, lay siege and force a peace treaty in which you ask for the city you want. This gives you zero extra warmonger amount, you only get the 250 for a DoW, which fades way in 50 turns.
