Major General McClellan.

My dear Sir.

Your despatches complaining that you are not properly sustained, while they do not offend me, do pain me very much.

Blencker's Division was withdrawn from you before you left here; and you knew the pressure under which I did it, and, as I thought, acquiesced in it -- certainly not without reluctance.

After you left, I ascertained that less than twenty thousand unorganized men, without a single field battery, were all you designed to be left for the defence of Washington, and Manassas Junction; and part of this even, was to go to Gen. Hooker's old position. Gen. Banks' corps, once designed for Manassas Junction, was diverted, and tied up on the line of Winchester and Strausburg, and could not leave it without again exposing the upper Potomac, and the Baltimore and Ohio Railroad. This presented, (or would present, when McDowell and Sumner should be gone) a great temptation to the enemy to turn back from the Rappahanock, and sack Washington. My explicit order that Washington should, by the judgment of all the commanders of Army corps, be left entirely secure, had been neglected. It was precisely this that drove me to detain McDowell.

I do not forget that I was satisfied with your arrangement to leave Banks at Mannassas junction; but when that arrangement was broken up, and nothing was substituted for it, of course I was not satisfied. I was constrained to substitute something for it myself. And now allow me to ask "Do you really think I should permit the line from Richmond, via Mannassas Junction, to this city to be entirely open, except what resistance could be presented by less than twenty thousand unorganized troops?" This is a question which the country will not allow me to evade.

There is a curious mystery about the number of the troops now with you. When I telegraphed you on the 6th. saying you had over a hundred thousand with you, I had just obtained from the Secretary of War, a statement, taken as he said, from your own returns, making 108,000 then with you, and en route to you. You now say you will have but 85,000, when all en route to you shall have reached you. How can the discrepancy of 23,000 be accounted for?

As to Gen. Wool's command, I understand it is doing for you precisely what a like number of your own would have to do, if that command was away.

I suppose the whole force which has gone forward for you, is with you by this time; and if so, I think it is the precise time for you to strike a blow. By delay the enemy will relatively gain upon you -- that is, he will gain faster, by fortifications and reinforcements, than you can by reinforcements alone.

And once more let me tell you, it is indispensable to you that you strike a blow. I am powerless to help this. You will do me the justice to remember I always insisted, that going down the Bay in search of a field, instead of fighting at or near Mannassas, was only shifting, and not surmounting, a difficulty -- that we would find the same enemy, and the same, or equal, intrenchments, at either place. The country will not fail to note -- is now noting -- that the present hesitation to move upon an entrenched enemy, is but the story of Manassas repeated.

I beg to assure you that I have never written you, or spoken to you, in greater kindness of feeling than now, nor with a fuller purpose to sustain you, so far as in my most anxious judgment, I consistently can. But you must act. Yours very truly