A messenger had galloped up to him and he brought formidable news.

A heavy column of horsemen had just appeared directly behind the Southern cavalry and was marching to the attack.

Stuart was in a trap.
Harry saw that Stuart had been outgeneralled, and again he shut his teeth together hard.

To be outgeneralled did not mean that they would be outfought.

The Northern force in their rear was the third division under Gregg, and Stuart sent back cavalry and guns to meet them.
Harry now saw the battle on all sides of him.