Each nation establishes conditions upon which aliens will be admitted, and makes laws concerning them. Most countries, including the United States, forbid or limit the admission of criminals, paupers, and the diseased. Certain groups and nationalities may be unconditionally excluded from legal residence, but such discrimination is likely to cause international friction.
Aliens, while they reside in a country, are subject to its laws and not to those of their home country, except in cases of extraterritoriality jurisdiction. A state distinguishes between aliens who are merely traveling or living there temporarily and those who have come to stay or work; wider powers are assumed over the second group. In the United States, permanent resident status may entitle an alien to a "green card," and thus to seek employment, and aliens have many of the privileges afforded to citizens, including public assistance and the right to attend public schools, even if they are in the country illegally. The Immigration and Nationality (McCarran-Walter) Act of 1952 requires aliens to register each year with the Bureau of Citizenship and Immigration Services (formerly the Immigration and Naturalization Service).
As citizens of another country, aliens may call on it to intercede in legal matters. Their home state may point out or protest injustice and may also threaten reprisals; such situations have frequently caused international disputes. On the other hand, aliens may find asylum in a country to which they have fled, unless treaties of extradition provide for their return. A state may for political or legal reasons expel an alien who has been admitted, by a procedure called deportation.
In time of war, laws governing aliens are usually stricter, and enemy aliens (nationals of enemy countries) may be restricted in various ways. Treaties between most governments provide for reasonable periods at the beginning of hostilities during which aliens may withdraw under supervision. World War II saw registration requirements and the exclusion of enemy aliens from certain U.S. areas, but the removal from their homes and internment in camps of Japanese on the West Coast applied to Americans of Japanese descent as well as to aliens.
Population and economic pressures periodically cause host countries to become less hospitable to aliens. In the 20th cent., U.S. immigration and deportation laws have changed in reaction to varying conditions. The "Red Scare" at the end of World War I brought a wave of deportations of anarchists and other radicals. A 1986 federal immigration bill, aimed at reducing the number of illegal aliens, allowed many of those who had been in the country for some time to apply for amnesty and stay legally, and millions took advantage.
In the 1990s politicians, including many in such states as California, where large numbers of illegal aliens reside, introduced state or federal legislation aimed at denying various privileges and benefits to aliens, in some cases even to those legally in the United States. A 1996 federal act led to a rapid increase in deportation of aliens who had not immigrated legally, and the INS grew to become the largest federal law-enforcement agency.
Illegal immigration has remained a contentious issue in the 21st cent., especially in those states bordering Mexico. Arizona voters, for example, passed (2004) a measure that requires public officials to turn in illegal immigrants who seek public services, including police and fire assistance, and in 2010 its legislature enacted a second measure that required local law officers to check the status of anyone they stopped who they believe might be an illegal alien. A number of other states have since adopted or considered laws intended to arrest or penalize illegal immigrants or those who hire or aid them, but a 2012 U.S. Supreme Court decision largely limited to the federal government the right to enact and enforce immigration law.