@Risely once rumors spread about the war being for nothing more commoners will join. War is always the toughest on commoners in these scenarios. Rebellion is usually pretty quick. Just depends on how many "men" were left behind (and weren't forced to join armies against their will)
Noble backing on the other hand will be difficult. Most likely a small portion will join early, some will join once the second prince gains more victory and others will only join when they get something for them.
Still be a tough road getting backing and supplies, though. Need food to keep an army going.