Sixth and Seventh Regiments (Colonel Edward Prince) of Illinois Cavalry under his command, since, by the most brilliant expedition of the war, they joined the forces under my command.

The moral effect of that remarkable expedition upon a wavering and astonished enemy, and the assistance rendered us in breaking up the enemy's communications, in establishing our own, and in covering the concentration of our forces against this place, can hardly be overestimated. Their timely presence has supplied a want which you will remember I have frequently represented was crippling all our operations.

I trust the services of Colonel Grierson and his command will receive at the hands of the Government that acknowledgment which they so eminently deserve.

I have the honor to be, general, very respectfully, your most obedient servant,

N. P. BANKS.

IN REAR OF Vicksburg, MISS., May 31, 1863.

(Received June 3-4 p. M.)

Major General N. P. BANKS, Comdg. Dept. of the Gulf:

GENERAL: Your letters of the 28th and 29th instant, by Colonel Riggin, have just been received. While I regret the situation in which they left you, and clearly see the necessity of you being re-enforced in order to be immediately successful, the circumstances by which I am surrounded will prevent my making any detachments at this time.

Concentration is essential to the success of the general campaign in the West, but Vicksburg is the vital, point. Our situation is for the first time during the entire Western campaign what it should be. We have, after great labor and extraordinary risk, secured a position which should not be jeopardized by any detachments whatever. On the contrary, I am now and shall continue to exert myself to the utmost to concentrate. The enemy clearly perceive the importance of dislodging me at all hazards. General Joe Johnston is now at Canton, organizing his forces and making his dispositions to attack me. His present strength is estimated at 40,000, and is known to be at least 20,000. The force he took from Jackson was 8,000; [W. W.] Loring's DIVISION, which has joined him since the battle of Champion's Hill, 3,000; General [S. R.] Gist, from South Carolina, 6,000; stragglers from Pemberton, 2,000; troops from Mobile, number not known. Besides this, Major-General Hurlbut writes me he is reliably informed that Bragg has detached three DIVISIONS from his army to report to Johnston. Pemberton has himself 18,000 effective men.

I have ample means to defend my present position, and effect the reduction of Vicksburg within twenty days, if the relation of affairs which now obtains remains unchanged. But detach 10,000 men from my command and I cannot answer for the result. With activity on the part of the enemy and any increase of his present force, it will become necessary for me to press my operations with all possible dispatch. I need not describe the severity of the labor to which my command must necessarily be subjected in an operation of such magnitude as that in which it is now engaged. Weakened by the detachment of 10,000 men, or even half that number, with the circumstances entirely changed, I should be crippled beyond redemption. My arrangements for supplies are ample, and can be expanded to meet any exigency. All I want now are men.

I am, sir, very respectfully, your obedient servant,

U. S. GRANT,