In Season 3 Episode 2, "Dark Wings, Dark Words", Lord Karstark tells Robb Stark:
I think you lost this war the day you married her [Talisa].
Why did he think so? I know Robb was supposed to marry one of Walder Frey's daughters, but what did he really lose by marrying Talisa?
He was promised to one of the Frey girls in exchange for crossing the bridge, but now that's irrelevant.
It also doesn't look like he had many of Frey's men to begin with, since in Season 3 Episode 5, "Kissed by Fire", he considers asking for their help to replace a chunk of his army. Additionally, it doesn't look like Frey's army is that large; Robb's army outnumbered Frey's five to one when they crossed Twins.
So why did Karstark believe the war was lost when Robb married Talisa?