He was likely going to have the backing of the largest Kingdom if he hadn't y'know, died. Robb was a very good battle commander, until he wasn't.

Like I said, it is powerful, but not so powerful to impose its will on everyone else. Even if Renly kills Stannis and Joffrey he still has to pacify implacably hostile forces in the Riverlands, the North, the Iron Islands, the Westerlands, and eventually Dorne.

i literally just said he had robb on his side? he wouldn't have to pacify the north if it became the six kingdoms

Yeah but uh, he doesn't want to be King of the Six Kingdoms, he wants all Seven. He tells Cat that the North cannot secede, which is Robb's entire goal. They are at an impasse, they have to fight or one of them has to back down.