We cannot count on keeping secret our intention to overthrow this force. Even if the many parties intrusted with the knowledge of the plan should not disclose or discover it, the necessary preliminary measures for such an expedition would betray it; and the are alive and well informed as to every movement, however slight, we make. They have, moreover, been expecting us to attack their position, and have been preparing for it. When it became known positively we are about to march, and they learn in what strength, they will be obliged to call in their disposable forces from all quarters, for they will not be able, if closely pressed, to get away by railroad before we can reach them. If General J. E. Johnston's force is kept engaged by Major-General Patterson, and Major-General Butler occupies the force now in his vicinity, I think they will not be able to bring up more than ten thousand men. So we must calculate on having to do with about thirty-five thousand men.

The objective point in our plan is the Manassas Junction. This is covered by the enemy's troops stationed at Centreville, Germantown, Fairfax Court-House, Fairfax Station, a place between Fairfax Station and Sangster's, and on the Occoquan. The position at Mannassas may be reached by four routes: First, by the Leesburg stone road, Georgetown turnpike, and Loudoun and Hampshire Railroad, via Falls Church and Vienna; second, by way of the Little River turnpike and Fairfax Court-House; third, by way of the Orange and Alexandria Railroad; fourth, by way of the road south of the Orange and Alexandria road.

There is a fifth approach, from Dumfries or Evansport, by way of Brentsvile, a march of about twenty-two miles, but the starting point is too far from the main direct approach to admit of its being used in the first instance without a superabundance of force. The country lying between the two armies is mostly thickly wooded, and the roads leading across it, except the turnpikes and railroads, are narrow, and in places sunken by the wear of travel and wash of rains. This makes it necessary to have the fewest possible number of carriages of any kind, and or forces, therefore, though the distance is short, will have to move over several lines of approach in order to get forward in time a sufficient body to operate with success. The Loudoun and Hampshire road is in working order as far as within five miles of Vienna, and no doubt could soon be repaired to that place. The Orange and Alexandria road, which I propose to look to as the main channel of supply, is now in working order some seven miles out of Alexandria, and from Manassas Junction to within fifteen miles of Alexandria. In the intermediate space the road has been destroyed as effectively as possible,and a long deep cut filled in with trees and earth. Nevertheless, all these obstacles can soon be removed with plenty of force and an adequate supply of proper materials.

Leaving small garrisons in the defensive works, I propose to move against Manassas with a force of thirty thousand of all arms, organized into three columns, with a reserve of ten thousand. One column to move from Falls Church of Vienna (preferably the latter), to go between Fairfax Court-House and Centreville, and, in connection with another column moving by the Little River turnpike, cut off or drive in (the former, if possible) the enemy's advanced posts. The third column to move by the Orange and Alexandria Railroad, and leaving as large a force as may be necessary to aid in rebuilding it, to push on with the remainder to join the first and second columns.

The enemy is said to have batteries in position at several places in his front, and defensive works on Bull Run and Manassas Junction. I