In customary international law, an enemy alien is any native, citizen, denizen or subject of any foreign nation or government with which a domestic nation or government is in conflict with and who are liable to be apprehended, restrained, secured and removed. Usually, but not always, the countries are in a state of declared war.
Enemy alien Wikipedia
At the outbreak of World War II, in 1939, the United Kingdom had become a place of refuge for people who had fled Nazi persecution, including Jews and political refugees. At first, the authorities interned these refugees with other enemy residents, without distinction. Later on, when Italy also declared war, significant numbers of Italian residents were also interned.
The Isle of Man, relatively isolated from the British mainland and with a useful amount of holiday accommodation was used to provide housing for the "Alien Civilians" (as it had in World War I). There were also efforts to move internees from Britain. In July, 1940, the Arandora Star was torpedoed and sunk while transporting Italian and German aliens to North America; 743 died, including prisoners, crew and guards. The 813 surviving prisoners were subsequently included in the 2,500 men transported by HMT Dunera for internment in Hay, New South Wales.
The Pioneer Corps was apparently the only British unit that enemy aliens could serve in early in the war. Many thousands of Germans and Austrians joined the Pioneer Corps to assist the Allied war efforts and liberation of their home countries. These were mainly Jews and political opponents of the Nazi Regime who had fled to Britain while it was still possible, and included the cinematographer Ken Adam, writer George Clare and publisher Robert Maxwell. These men - often dubbed "The King's Most Loyal Enemy Aliens" - later moved on to serve in fighting units. Some were recruited by Special Operations Executive (S.O.E.) as secret agents. They were instructed to choose an "English" name using their old initials.
Serving as German nationals in the British forces was particularly dangerous, since, in case of taken captive, with a high probability they would have been executed as traitors by the Germans. Also, the number of German-born Jews joining the British forces was exceptionally high; by the end of the war, one in seven Jewish refugees from Germany had joined the British forces. Their profound knowledge of the German language and customs proved useful. A lot of them served in the administration of the British occupation army in Germany and Austria after the war.
A well-known example of enemy aliens were the Japanese citizens residing in the United States during World War II. Many of these Japanese and Japanese Americans were imprisoned in internment camps by President Roosevelt during wartime, alongside many German- and Italian-Americans. However, many Japanese Americans and Italian-Americans were not actually "aliens", as they held American citizenship. The term "enemy alien" referred only to non-American citizens who were nationals of Axis countries. Included in this number were thousands of resident aliens who were prohibited from applying for citizenship by race-based naturalization laws; when war was declared against their native countries, their status changed from "resident" to "enemy" alien. Therefore, German American, Italian American and Japanese American permanent residents were classified as enemy aliens and interned as such. In total 10,905 Italian Americans and approximately 110,000 Japanese Americans were interned in many different camps and sites across the country. German Americans were held in more than 50 different locations.
Citizens of an enemy country who lived in the USA during World War II, were required to have a "Enemy Alien" card and register monthly with the authorities. Similar regulations existed in Canada and Mexico.