The question respecting slavery in the old thirteen States had been decided and settled before the adoption of the Constitution, which grants no power to Congress to interfere with, or to change what had been so previously settled.  The slave States, therefore, are free to continue or to abolish slavery.  Since the year 1808 Congress have possessed power to prohibit and have prohibited the further migration or importation of slaves into any of the old thirteen States, and at all times, under the Constitution, have had power to prohibit such migration or importation into any of the new States or territories of the United States.  The Constitution contains no express provision respecting slavery in a new State that may be admitted into the Union; every regulation upon this subject belongs to the power whose consent is necessary to the formation and admission of new States into the Union.  Congress may, therefore, make it a condition of the admission of a new State, that slavery shall be forever prohibited within the same.  We may, with the more confidence, pronounce this to be the true construction of the Constitution, as it has been so amply confirmed by the past decisions of Congress.

Although the articles of confederation were drawn up and approved by the old Congress, in the year 1777, and soon afterwards were ratified by some of the States, their complete ratification did not take place until the year 1781.  The States which possessed small and already settled territory, withheld their ratification, in order to obtain from the large States a cession to the United States of a portion of their vacant territory.  Without entering into the reasons on which this demand was urged, it is well known that they had an influence on Massachusetts, Connecticut, New York, and Virginia, which States ceded to the United States their respective claims to the territory lying northwest of the river Ohio.  This cession was made on the express condition, that the ceded territory should be sold for the common benefit of the United States; that it should be laid out into States, and that the States so laid out should form distinct republican States, and be admitted as members of the Federal Union, having the same rights of sovereignty, freedom, and independence as the other States.  Of the four States which made this cession, two permitted, and the other two prohibited slavery.

The United States having in this manner become proprietors of the extensive territory northwest of the river Ohio, although the confederation contained no express provision upon the subject, Congress, the only representatives of the United States, assumed as incident to their office, the power to dispose of this territory; and for this purpose, to divide the same into distinct States, to provide for the temporary government of the inhabitants thereof, and for their ultimate admission as new States into the Federal Union.

The ordinance for those purposes, which was passed by Congress in 1787, contains certain articles, which are called “Articles of compact between the original States and the people and States within the said territory, for ever to remain unalterable, unless by common consent.”  The sixth of those unalterable articles provides, “that there shall be neither slavery nor involuntary servitude in the said territory.”