Let's look at a few other considerations. In this post, I'll review the relevant "legislative history" of the Constitutional Convention. Current original understanding theory holds that the unexpressed and (at the time of ratification) unknown intent of the members of the Constitutional Convention is irrelevant. Still, it's fun to investigate.
Ironically, the original version of the Clause contained language that provided more support for the suggestion that new states could be formed only from then-existing territory of the United States.
The admission of additional states was raised as an issue at the very outset of the constitutional convention. On Tuesday May 29, 1787, Edmund Randolph of Virginia presented, on behalf of the Virginia delegation, a series of 15 resolutions commonly known as the Virginia Plan. The 10th resolution addressed the admission of new states:
10. Resolvd. that provision ought to be made for the admission of States lawfully arising within the limits of the United States, whether from a voluntary junction of Government & Territory or otherwise, with the consent of a number of voices in the National legislature less than the whole.
So far as I can tell, it is impossible to be sure whether the phrase “within the limits of the United States” was intended to imply the limits of the United States as they then existed in 1787. But at least the phrase lends itself to that possibility.
On Tuesday June 5, 1787, the 10th resolution came up for discussion before the convention sitting as a committee of the whole. The discussion was brief:
Resolution 10 was agreed to-viz-that provision ought to be made for the admission of States lawfully arising within the limits of the U. States, whether from a voluntary junction of Government & territory, or otherwise, with the consent of a number of voices in the National Legislature less than the whole.
On Thursday July 26, 1787, the Convention referred a series of resolutions to the Committee of Detail. The resolution concerning new states, then numbered XVII, was at that point unchanged:
XVII. RESOLVED, That provision ought to be made for the admission of states lawfully arising within the limits of the United States, whether from a voluntary junction of government and territory, or otherwise, with the consent of a number of voices in the national legislature less than the whole.
On Tuesday August 6, 1787, John Rutledge of South Carolina delivered the Report of the Committee of Detail. The Committee’s version of Resolution XVII, now labeled Article XVII, continued to include the “within the limits of the United States” language:
New States lawfully constituted or established within the limits of the United States may be admitted, by the Legislature, into this Government; but to such admission the consent of two thirds of the members present in each House shall be necessary. If a new State shall arise within the limits of any of the present States, the consent of the Legislatures of such States shall be also necessary to its admission. If the admission be consented to, the new States shall be admitted on the same terms with the original States. But the Legislature may make conditions with the new States, concerning the public debt which shall be then subsisting.
On Wednesday August 29, the Committee of Detail’s draft of Article XVII came up for review before the convention. For reasons that Madison does not explain, the convention voted unanimously to replace the first clause with new language that did not include the phrase “within the limits.” The relevant portion of Madison’s notes reads as follows in its entirety:
Mr. Govr. MORRIS moved the following proposition as a substitute for the XVII art:
"New States may be admitted by the Legislature into this Union: but no new State shall be erected within the limits of any of the present States, without the consent of the Legislature of such State, as well as of the Genl. Legislature"
The first part to Union inclusive [that is, the first clause, “New States may be admitted by the Legislature into this Union”] was agreed to nem: con: [without dissent].
Although there was a good deal of discussion about the second part of the sentence – the circumstances under which new states could be created out of present states, in whole or in part – the initial clause remained the same.
On Saturday September 15, the delegates reviewed the constitution and considered final amendments. The Section concerning the admission of new states had already reached its final form:
Art. IV. Sect 3. "New States may be admitted by the Congress into this Union: but no new State 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 Congs"