A more thorough explanation:

An alien enemy is a citizen or subject of a country that is at war with the country in which they are living or traveling. They are also known as an enemy alien. This term is used in the context of civil rights enforcement, and it is not based on nationality but on residence or place of business.

For example, if a person is a British subject or the subject of a neutral state but is voluntarily living or doing business in an enemy country, they are considered an alien enemy. This means that they may be subject to certain restrictions or even detention during times of war.

Another related term is illegal alien, which refers to an alien who enters a country at the wrong time or place, eludes officials, or enters into a sham marriage to evade immigration laws. This is different from a nonresident alien, who is simply a person who is neither a resident nor a citizen of a country.

Overall, the term alien enemy is used to identify individuals who may pose a threat to national security during times of war or conflict.