Final answer:
To apply for naturalization in the United States, a non-citizen must legally reside in the country for at least five years. This requirement, once fourteen years, has long been reduced. Special provisions exist for non-citizens in the military.
Step-by-step explanation:
The current United States law requires that a non-citizen must reside in the country for at least five years before they can apply for naturalization as a citizen. This duration was originally fourteen years under the Alien and Sedition Acts of 1798, but was eventually reduced. Now, to apply for naturalization, an individual must complete a 21-page application form, the N-400, where they provide various details including personal information, proof of residency, attachment to the principles of the Constitution, and renunciation of previous allegiances.
Notably, there have been changes to make it easier for non-citizens serving in the United States military to become naturalized. To gain a deeper understanding of the naturalization process, it's beneficial to review the different laws regarding naturalization and immigration from the establishment of the Naturalization Act of 1790 to present times.