A person permanently barred from obtaining US citizenship is inadmissible. This category of people primarily includes people who got out of military service based on their alienage, and people who left the US to avoid the draft. It also includes those that have been previous removed (deportation and exclusion) unless the required time is spent outside the U.S. before they reenter.
Note that aliens who have been deported are inadmissible. After a first deportation, the person is inadmissible for five years, and after subsequent deportations, the period of inadmissibility is 20 years. A person deported because of an aggravated felony is permanently inadmissible. People who have been unlawfully present in the US for more than 180 days but less than a year are inadmissible for three years. Unlawful presence of more than a year leads to inadmissibility for ten years.
A person is also inadmissible if it is determined that they are required to assist another person who is inadmissible. Persons who have detained a US citizen child outside the US are inadmissible until they comply with any court order regarding the child’s custody. Persons coming to the US to engage in polygamy are inadmissible. Also, former US citizens who renounced their citizenship for tax purposes are inadmissible