So, so foolish of you Robb, and arguably one of the most outright, stupid moves in the entire series. After swearing to marry one of Walder Freys daughters, on oath, Robb then goes and backhandedly marries a healer. Not only is this insulting to the Freys as its a serious oath to break, but hes also marrying someone who is essentially a nobody. While she is technically a noblewomen, her status is quite diminished in Westeros, where she works selflessly as a healer. Naturally, this leads to a severe loss of troops and support from the Freys, further contributing to their betrayal. Had Robb not married Talisa, perhaps he could have held on to the additional troops long enough to win the war. Theres still no accounting for what may have happened with the Karstarks though, a mistake, which, compounded with his marriage to Talisa, greatly influenced the Red Wedding.