Clause 1. New States may be admitted by the Congress into this Union; but no new States shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
The Framers of the Constitution planned for the future growth of the country, setting up a system to allow new states formed on the western frontier to enter the Union as equals of the original states. Imagine how different America would look today if the original thirteen states had decided to treat the western settlements as colonies instead.
Clause 2. The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
Any western territories that haven't yet become states fall under the direct control of the Congress.