According to the United States Immigration and Naturalization Act (INA), an individual who seeks to obtain US citizenship must meet a variety of eligibility requirements and the first thing that a foreign national must do in order to obtain US citizenship, is to become a Green Card holder. A person must prove that he has lived in the United States as a permanent resident for a five year period and he must possess good moral character, in order to apply for US citizenship. But the spouses of US citizens need not wait for five years and they can apply for US citizenship if they have maintained permanent resident status for a three year period. To become eligible for US citizenship through marriage, you will have to demonstrate that you are legally married to a US citizen and you must hold a valid marriage certificate. Likewise, your US citizen spouse also must demonstrate that he is a US citizen and provide few supporting documents such as copies of his US passport, citizenship certificate or naturalization certificate, as proof of US citizenship status.
You will have to prove that your marriage is valid under the law of the country in which the marriage was performed and that the marriage law of the jurisdiction does not conflict with the federal marriage laws. United States recognizes marriages that were performed in foreign countries but it is the responsibility of the applicant to establish that her marriage is valid. However, USCIS may not recognize bigamy, polygamy, proxy marriages and relationships between two people of the same sex. However, USCIS will recognize marriages that were performed in states or jurisdictions that recognize heterosexual marriages and in states that do not bar marriages between transgender persons and persons of other genders. Likewise, USCIS will recognizes common law marriages, in certain circumstances.
Your US citizen spouse must remain a US citizen, until you take the Oath of Allegiance and if your spouse loses his US citizenship status before you become a naturalized US citizen, you will become ineligible for US citizenship. You will have to establish that you are living in marital union with your US citizen spouse and you may become ineligible for naturalization if you are not residing with your US citizen spouse at the time of filing your naturalization application. Likewise, if you were separated from your spouse, prior to taking the oath, you may not qualify for US citizenship. However, to qualify for naturalization, you will have to demonstrate that you have lived with your US citizen spouse for the required period of time.
You cannot swear the Oath of Allegiance, if your US citizen spouse dies before you become a naturalized US citizen. But this does not apply for the spouses of US citizens who die during a period of service in the US armed forces and spouses of such US citizens, can become naturalized US citizens under a special provision. If your marital status was terminated, you can no longer apply for US citizenship as the spouse of a US citizen and you cannot swear the oath and obtain a naturalization certificate, if the divorce or annulment occurs before you take the oath. If you are formally separated from your spouse and if you have not lived in marital union with your US citizen spouse during the time period in which you should have lived with your US citizen spouse, you cannot apply for US citizenship as the spouse of a US citizen.