Mr. S. D. Allen writes from Alexandria, La., that the people despair of defending the Mississippi Valley with such men as Pemberton and other hybrid Yankees in command. He denounces the action also of quartermasters and commissaries in the Southwest.

A letter from Hon. W. Porcher Miles to the Secretary of War gives an extract from a communication written him by Gen. Beauregard, to the effect that Charleston must at last fall into the hands of the enemy, if an order which has been sent there, for nearly all his troops to proceed to Vicksburg, be not revoked. There are to be left for the defense of Charleston only 1500 exclusive of the garrisons!

May 15th.—The Tredegar Iron Works and Crenshou’s woolen factory were mostly destroyed by fire last night! This is a calamity.

We have also intelligence of the occupation of Jackson, Miss., by the enemy. Thus they cut off communication with Vicksburg, and that city may be doomed to fall at last. The President is at work again at the Executive Office, but is not fully himself yet. The Secretary of War dispatched Gen. Lee a day or two ago, desiring that a portion of his army, Pickett’s division, might be sent to Mississippi. Gen. Lee responds that it is a dangerous and doubtful expedient; it is a question between Virginia and Mississippi; he will send the division off without delay, if still deemed necessary. The President, in sending this response to the Secretary, says it is just such an answer as he expected from Lee, and he approves it. Virginia will not be abandoned.

Gens. Lee, Stuart, and French were all at the War Department to-day. Lee looked thinner, and a little pale. Subsequently he and the Secretary of War were long closeted with the President.

Gen. Schenck (Federal) has notified Gen. W. E. Jones, that our men taken dressed in Federal uniform will not be treated as prisoners of war, but will be tried and punished as spies, etc. The President directed the Secretary of War to-day to require Gen. Lee to send an order to the commander of the Federal army, that accouterments and clothing will be deemed subjects of capture, and if our men are treated differently than prisoners of war, when taken, we will retaliate on the prisoners in our possession.

Gen. Longstreet censured Gen. French for his conduct before Suffolk, and the Secretary of War proposed that French be relieved, and sent before a court of inquiry. The President vetoed this, saying such courts were nuisances, and would not have him molested at this critical moment.

Gen. D. H. Hill writes that desertions in North Carolina are alarmingly frequent; that deserters will soon be in arms; that papers and factions exist there in favor of reconstruction, laboring to convince the people that the State has been neglected by the Confederate States Government, and he suggests summary punishments. The President directs the Secretary to correspond with Gov. Vance on the subject.

Mr. Benjamin has had some pretty passports printed. He sends one to Assistant Secretary Campbell for a Mr. Bloodgood and son to leave the Confederate States. I hope there is no bad blood in this incessant intercourse with persons in the enemy’s country. Just at this crisis, if so disposed, any one going thither might inflict incalculable injury on the cause of Southern independence.

May 16th.—It appears, after the consultation of the generals and the President yesterday, it was resolved not to send Pickett’s division to Mississippi, and this morning early the long column march through the city northward. Gen. Lee is now stronger than he was before the battle. Gen. Pickett himself, with his long, black ringlets, accompanied his division, his troops looking like fighting veterans, as they are. And two fine regiments of cavalry, the 2d and 59th North Carolina Regiments, passed through the city this morning likewise.