It is also so powerful as all you have to do is get declared on and then sign a peace/alliance combo with another civ. If they break treaty then when you make peace with other civ you can ally against the alliance breaking civ giving a second chance at war happiness.
At least that is how i understand it.
Rather than having to wait for the AI to DoW you, you can jump-start the process a bit. Make a straight-up war declaration on some distant or weak civ, and then use peace renegotiations to sign your target's neighbors in on the war. If any neighbors have the decency

to sign peace before the 20 turns expire, then you get war happiness from any who do.
The downside of tying peace deals with MAs in that you cannot control when you get declared on. The AI can break the deal at any random time and this can be quite inconvenient. As well, you sometimes have to be careful to not do too many of these deals at the same time. If a bunch of peace deals are broken at the same time, you might get suddenly dogpiled.
One thing I have found, too, is to
not tie together peace and an MA with some civ that you have recently gotten a lot of war weariness from. Normally, when you get a lot of war weariness from some civ, after 20 turns of peace all the weariness is erased from your people's minds. But if you do include a MA when you sign peace, and then that civ breaks the MA deal and you are back at war with them, all that war weariness comes rushing back even though *technically* you have war
happiness now from that civ. Basically, the new-found war happiness just reduces the war weariness, and if that war weariness was really high before, it will still be really high. This is all pretty obvious, really, but I made this mistake a couple of times when I first started using MAs and peace deals together.
Oh, and mc-red, I played Civ for 2+
years before I discovered that active deals button. It was a rather "Eureka!" moment when I did. I had heard about the button, but was too embarressed to ask such a "dumb" question.
