WASHINGTON, July 19, 1864.

Lieutenant-General GRANT,

City Point:

GENERAL: The recent raid into Maryland seems to have established several things, which it would be well for us to keep in mind:

First. It has proved that while your army, which comprises all troops north of Richmond that can go into the field, is entirely to weak to hold West Virginia and the Baltimore and Ohio Railroad, and at the same time to resist any considerable rebel raid north of the Potomac.

Third. We cannot rely upon aid from the militia of the Northern States. They will not come out at all, or will come too late, or in so small a force as to be useless.

Fourth. The garrisons of Washington and Baltimore are made up of troops entirely unfit for the field and wholly inadequate for the defense of these places. Had it not been for the opportune arrival of the veterans of the Sixth Corps both cities would have been in great danger. So long as you were operating between Washington and the enemy your army covered Maryland and Pennsylvania, and I sent you all the troops from here and the North which could take the field or guard your deport and prisoners of war. But the circumstance have now most materially changed, and I am decidedly of opinion that a larger available force should be left in this vicinity.

It may be answered that re-enforcements can be sent in time from the James River, as was done in this case. This answer would be decisive, if we here, or your there, could always be apprised of the number and position of the raiders, as well as the object upon which their march is directed. But this cannot be done without a superior cavalry force, which we have not got and are not likely to have. The country is so stripped of animals that it is hardly possible to supply demands in the field. If the enemy had crossed the Potomac below Harper's Ferry (and it is now fordable in many places), and had moved directly upon Washington of Baltimore, or if the arrival of the Sixth Corps had been delayed twenty-four hours, one or the other of these places, with their large depots of supplies, would have been in very considerable danger. Will it be safe to have this risk repeated? Is not Washington too important in a political as well as a military point of view to run any serious risk at all? I repeat that so long as Lee is able to make any large detachments Washington cannot be deemed safe without a larger and more available force in its vicinity.

What you say of establishing schools of instruction here, at Baltimore, and at Harper's Ferry will be applicable when we get troops to be instructed. But we are now not receiving one-half as many as we are discharging. Volunteering has virtually ceased, and I do not anticipate much from the President's new call, which has the disadvantage of again postponing the draft for fifty days. Unless our Government and people will come square up to the adoption of an efficient and thorough draft we cannot supply the waste of our army.

Very respectfully, your obedient servant,

H. W. HALLECK,

Major-General and Chief of Staff.