You may become a citizen in one of three ways:
Birth in the US – The 14th Amendment to the Constitution provides that anyone born in the US is a citizen of the US even if they hold dual citizenship. Therefore, children born in the US are citizens whether their parents are citizens, permanent residents, temporary visa holders or illegal aliens. The only children born in the US to whom this rule does not apply are the children of foreign diplomats.
Naturalization – In general, you must satisfy the five following requirements in order to become a citizen through naturalization:
Residency – Most persons must first attain permanent residence before applying for naturalization. The primary exception to this rule are persons who served in the U.S. armed forces during a period of hostilities designated by the President.
You must be a permanent resident for five years before becoming naturalized although the law permits you to apply for naturalization 90 days prior to completing the residency period.
If you are married to a US. citizen, you may be eligible for naturalization within three years if you have been married to a US citizen for three years, your spouse has been a citizen for the entire three-year period, and you are living in “marital unity”.
If you are a member of the US armed forces, you may be eligible to naturalize without any specific period of residence if you are a permanent resident, you have served on active duty for a period of three years or more, and you are serving honorably, or were given an honorable discharge. If you have been discharged, you must apply for naturalization within six months to take advantage of this rule.
If you are the spouse of a US citizen who is assigned to work abroad by the US government or by certain designated companies or organizations, residency rules may not apply to you.
Departure from the US for a period of six months or more creates a rebuttable presumption that you have abandoned your residency. A departure for one year or more creates a conclusive presumption (not subject to appeal that you have abandoned your residency. Some people who obtain Re-Entry Permits in order to exit the US for more than one year may preserve their residency but may still break their residency for naturalization purposes unless they take further steps to preserve it.