HEADQUARTERS OF THE ARMY, Washington, February 23, 1862.

GENERAL: You are assigned to the command of the land forces destined to co-operate with the Navy in the attacks upon New Orleans. You will use every means to keep your destination a profound secret, even from your staff officers, with the exception of your chief of staff and Lieutenant Weitzel, of the Engineers. The force at your disposal will consist of the first thirteen regiments named in your memorandum handed to me in person, the Twenty-first Indiana, Fourth Wisconsin, and Sixth Michigan (old and good regiments from Baltimore).

The Twenty-first Indiana, Fourth Wisconsin, and Sixth Michigan will await your orders at Fort Monroe.

Two companies of the Twenty-first Indiana are well drilled as heavy artillery. The cavalry force already en route for Ship Island will be sufficient for your purposes..

After full consultation with officers well acquainted with the country in which it is proposed to operate, I have arrived at the conclusion that two light batteries fully equipped and one without horses will be all that are necessary. This will make your force about 14,400 infantry, 275 cavalry, 580 artillery; total, 15,255 men. The commanding general of the Department of Key West is authorized to loan you, temporarily, two regiments; Fort Pickens can, probably, give you another, which will bring your force to near 18,000.

The object of your expedition is one of vital importance-the capture of New Orleans. The route selected is up the Mississippi River, and the first obstacle to be encountered (perhaps the only one) is in the resistance offered by Forts Saint Philip and Jackson. It is expected that the Navy can reduce these works; in that case you will, after their capture, leave a sufficient garrison in them to render them perfectly secure; and it is recommenced that, on the upward passage, a few heavy guns and some troops be left at the pilot station (at the forks of the river) to cover a retreat in the event of a disaster. These troops and guns will of course be removed as soon as the forts are captured. Should the Navy fail to reduce the works, you will land your forces and siege train, and endeavor to breach the works, silence their fire, and carry them by assault.

The next resistance will be near the English Bend, where there are some earthen batteries. Here is may be necessary for you to land your troops and co-operate with the naval attack, although it is more than probable that the Navy unassisted can accomplish the result. If these works are taken, the city of New Orleans necessarily falls. In that event, it will probably be best to occupy Algiers with the mass of your troops, also the eastern bank of the river above the city. It may be necessary to place some troops in the city to preserve order, but if there appears to be sufficient Union sentiment to control the city, it may be best for purposes of discipline to keep your men out of the city.

After obtaining possession of New Orleans, it will be necessary to reduce all the works guarding its approaches from the east, and particularly to gain the Manchac Pass.

Baton Rouge, Berwick Bay, and Fort Livingston will next claim your attention.

A feint on Galveston may facilitate the objects we have in view. I need not call your attention to the necessity of gaining possession of all the rolling stock you can on the different railways and of obtaining control of the roads themselves. The occupation of Baton Rouge by a combined naval and land force should be accomplished as soon as possible after you have gained New Orleans. Then endeavor to open your communication with the northern column by the Mississippi, always bearing is mind the necessity of occupying Jackson, Miss., as soon as you can safely do so, either after or before you have effected the junction. Allow nothing to divert you from obtaining full possession of all the approaches to New Orleans. When that object is accomplished to its fullest extent, it will be necessary to make a combined attack on Mobile, in order to gain possession of the harbor and works, as well as to control the railway terminus at the city. In regard to this I will send more detailed instructions as the operations of the northern column develop themselves.

I may briefly state that the general objects of the expedition are, first the reduction of New Orleans and all its approaches; then Mobile and its defenses; then Pensacola, Galveston, &c. It is probable that by the time New Orleans is reduced it will be in the power of the Government to re-enforce the land forces sufficiently to accomplish all these objects. In the mean time you will please give all the assistance in your power to the army and navy commanders in your vicinity, never losing sight of the fact that the great object to be achieved is the capture and firm retention of New Orleans.

I am, &c.,
GEO. B. McCLELLAN,

Major-General, Commanding U. S. Army.

Major General B. F. BUTLER, U. S. Volunteers.