Their use in the various forms of work to which they were allotted, and for which they were admirably qualified, released the same number of white men, who could at once be mustered into the ranks.

The slaves were therefore an effective addition to the military strength of the Confederacy from the very beginning of the war, and had seriously increased the available force of fighting men at the first engagement between the two armies.
As soon as this fact became well established, Congress proceeded to enact the first law since the organization of the Federal Government by which a slave could acquire his freedom.

The "Act to confiscate property used for insurrectionary purposes" was on the calendar of the Senate when the disaster at Bull Run occurred, and had been under consideration the day preceding the battle.

As originally framed, it only confiscated "any property used or employed in aiding, abetting, or promoting insurrection, or resistance to the laws." The word "property" would not include slaves, who, in the contemplation of the Federal law, were always "persons." A new section was now added, declaring that "whenever hereafter during the present insurrection against the Government of the United States, any person held to labor or service under the law of any State shall be required or permitted by the person to whom such labor or service is due to take up arms against the United States, or to work in or upon any fort, dock, navy-yard, armory, intrenchment, or in any military or naval service whatever against the Government of the United States, the person to whom such service or labor is due shall forfeit his claim thereto." The law further provided in effect that "whenever any person shall seek to enforce his claim to a slave, it shall be a sufficient answer to such claim, that the slave had been employed in the military or naval service against the United States contrary to the provisions of this Act." ZEAL AND INDUSTRY OF CONGRESS.
The virtue of this law consisted mainly in the fact that it exhibited a willingness on the part of Congress to strike very hard blows and to trample the institution of slavery under foot whenever or wherever it should be deemed advantageous to the cause of the Union to do so.

From that time onward the disposition to assail slavery was rapidly developed, and the grounds on which the assurance contained in the Crittenden Resolution was given, had so changed in consequence of the use of slaves by the Confederate Government that every Republican member of both Senate and House felt himself absolved from any implied pledge therein to the slave-holders of the Border States.