There he formed line of battle, and waited.
It is obvious that, under these circumstances, the true policy of General Pope was to obstruct Thoroughfare Gap, the only road by which Lee could approach promptly, and then crush Jackson.

On the night of the 27th, General McDowell was accordingly sent thither with forty thousand men; but General Pope ordered him, on the next morning, to Manassas, where he hoped to "bag the whole crowd," he said--that is to say, the force under Jackson.

This was the fatal mistake made by General Pope.

Thoroughfare Gap was comparatively undefended.

While General Pope was marching to attack Jackson, who had disappeared, it was the next thing to a certainty that General Lee would attack _him_.
All parties were thus moving to and fro; but the Confederates enjoyed the very great advantage over General Pope of knowing precisely how affairs stood, and of having determined upon their own plan of operations.