Interesting that the Founders put Naturalization in the same section as bankruptcies.
There were questions about simply granting citizenship to those French and Spanish citizens living in the Louisiana Purchase area. I have not seen any mention of Indians, but the Constitution did not consider them citizens at the time, because they were not taxed.
"Many Americans supported the purchase because it meant that they would be able to expand the country by moving further west. There were several Federalists that disagreed with the purchase and said that Jefferson was undermining the Constitution. The Constitution did not mention the purchase of territory. Even Jefferson questioned if the Constitution allowed the government to add territory and grant citizenship to the roughly 50,000 people that lived in the Louisiana Territory. Previously, Jefferson supported a strict interpretation of the Constitution. Jefferson and Congress both believed that the Louisiana Purchase was beneficial to the country, so they agreed not to worry about any constitutional difficulties." http://www.lessonsnips.com/docs/pdf/louisianapurchase.pdf
Article III of the Louisiana Purchase Treaty Says:
"The inhabitants of the ceded territory shall be incorporated in the Union of the United States and admitted as soon as possible according to the principles of the federal Constitution to the enjoyment of all these rights, advantages and immunities of citizens of the United States, and in the mean time they shall be maintained and protected in the free enjoyment of their liberty, property and the Religion which they profess."
I'm not a lawyer, but this paragraph seems to say that the inhabitants could be made citizens when they went through the Naturalization process Congress had established under the authority granted them by the Constitution, rather than by a blanket process covering them all at once. French law, until repealed, had a provision that allowed inhabitants of former French territories to become citizens of France without waiting five years, as was the norm. The US government allows members of our armed forces who are not citizens to utilize a streamlined application process after serving three years. There is one case where a Marine who was a Mexican citizen, but who wanted to become an American, was killed in Iraq. Congress passed a special bill to grant him citizenship postumously. I haven't researched this, but perhaps Congress, either formally or informally, may have done something like this to cover the inhabitants of Louisiana. ( And perhaps former Confederates born during the war.) Experts? Stan