He saw at once that his communications with Richmond, about which he was so solicitous, were greatly endangered, for the Union army could be formed to interpose between him and Williamsport, and still keep a safe line of retreat open to Washington.

This might not be so great a misfortune to the enemy as regards food and forage; for he could probably live on the country for some time, by making predatory excursions in different directions, but when it came to obtaining fresh supplies of ammunition, the matter would become very serious.

An army only carries a limited amount of this into the field and must rely upon frequent convoys to keep up the supply, which is constantly decreasing from the partial engagements and skirmishes, so prevalent in a hostile country.
The wisdom of Hooker's policy in desiring to assail the rebel communications is demonstrated by the fact the Lee immediately turned back.

The head of the serpent faced about as soon as its tail was trodden upon.

He came to the conclusion to prevent an attack against his rear by threatening Baltimore with his whole force.