In law, an alien is a foreign-born resident who has not been naturalized and is still a citizen or subject of a foreign country, although definitions and terminology differ across legal systems.
World War II poster
People of various background became naturalized at Kennedy Space Center in Florida (2010). Before the naturalization they were lawfully admitted permanent resident aliens.
In customary international law, an enemy alien is any alien native, citizen, denizen or subject of any foreign nation or government with which a domestic nation or government is in conflict and who is
Alien (law)
…such as a travel visa holder or a foreign tourist, who is lawfully within a nation but whose legal domicile is in another nation. alien enemy (or enemy alien) — any foreign national of any country that is at war with the host country. undocumented alien (or removable alien) — any person who has been found…
Family held at Tatura Internment Camp 3, Australia, in 1943