If a citizen of the United States has a fiancé who is a citizen of a different country, the fiancé will need to apply for a K-1 visa in order to marry in the U.S. To apply, there are certain qualifications that the couple needs to meet. 

According to the U.S. Department of State, one of the requirements is the couple needed to personally meet within the prior two years, unless the fiancé lives in a culture that does not allow a man and woman to see each other prior to marriage. Upon approval of the visa, the fiancé may come to the U.S., and the marriage must occur within 90 days of the fiancé arriving in the country. After marriage, the foreign citizen may begin the process of applying for a Green Card. 

According to the U.S. Citizenship and Immigration Services, the fiancé is not eligible for a K-1 visa if he or she already lives in the U.S. or if the couple is planning on marrying somewhere other than the United States. 

The first step to obtaining a K-1 visa is for the U.S. citizen to file Form I-129F. When approved, the DOS National Visa Center forwards the form to the consulate in the country where the fiancé lives so he or she can apply for the visa. There will be an interview at the consulate for the fiancé, and the fiancé needs to bring all required information to the interview. 

If the officer approves the visa, it is good for one entry into the United States and is valid for up to six months. Upon arrival at a port of entry, a CBP officer will decide whether or not to allow the fiancé into the country. If allowed, and the marriage occurs within 90 days, the new spouse may file Form I-485 to begin the process of obtaining a Green Card.