Yet the Klingons aren't in a bad shape or Section 31 wouldn't be worried about them. I guess they conquered worlds with energy resources as result of Nero's appearence.
Or it could be that Klingons are no threat at all to the Federation, but rather a perpetual nuisance, and Section 31 would prefer for the Federation to conquer/reeducate/civilize them than continue to put up with border raids, kidnappings and murders year after year. Basically, the Taliban with starships: they keep showing up in random bursts of spectacular violence that cause a lot more dismay than actual damage. Marcus' idea is that the best way to deal with the Klingons is to roll in there, kick their asses and force them to behave themselves.
It's basically what Cartwright was saying in TUC. "The opportunity here is to bring them to their knees." The key word here is opportunity
: it's something that some in Starfleet always wanted to do if they could just think of a good enough excuse.