[Man of the North]

Robb Stark had better training as a military leader and better disciplined army to boot. Renly may be older than Robb but never had the military training. Renly thought it was a game were he was going to march north from Stormend and smash their opponent. But Renly did have the numbers, considering his force was over a hundred thousand strong, no opponent would last very long in that fight. He got most of those troops from the Tyrells because he married their daughter. So he was a better politician than Robb ever was, because he marriage give him an army.

Stannis army was made mostly of lords that were loyal to Stormsend not Stannis, but with the death of Renly, their loyalties goes to Stannis, since he is the rightful heir. Stannis should have figured out what the Tyrells army was doing before he made his advancement towards KL.

But there is a point in lords switching sides so quickly. They should not be trusted, which is why Stannis should have figured out what was going on with the Tyrells first. Then he would have some time to valuate his army's strength and weaknesses.