It is possible for foreign persons to obtain American citizenship if they are employed in the U.S. army. It is also important to know that the family members of the persons performing military services for the U.S. army can obtain American citizenship.  If you are employed in the U.S. army or in other similar institutions, such as the U.S. Navy or the National Guard, you will be required to apply for a visa at the U.S. Citizenship and Immigration Services (USCIS), as long as you have been working in the American institution for minimum one year (during peacetime conditions).