Immigration Detention is the policy of holding those whom law enforcement believes may be guilty of illegally entering a country. In the United States, this is popularly known as Border Detention, because much of it takes place on or near the southern border with Mexico.
Most countries who are destination for immigration have detention centers where they hold migrants until immigration authorities either decide to grant them a visa and release them or decline to grant them a visa and deport them to their countries of origin. Australia is notable for having outsourced these centers to remote Pacific islands where there is almost no chance that a potential migrant would be able to escape into the wider Australian population, but most countries' detention centers lie within their own borders. Some countries allow indefinite detention, while others set strict limits for how long a person may be detained. Wherever they are, detention centers have a reputation for human rights abuses that the respective governments have found hard to shake.
Immigration detention in the United States officially began with the opening of Ellis Island in 1890. Every immigrant coming to America through the Port of New York had to be processed on the island. Many only spent a few hours, but others were detained and held overnight or even longer if they or a member of their family was sick or had heath problems or there were legal issues with their arrival. Most the most common detainees were women travelling alone or with children. Beginning in the 1920s, changes to American law decreased immigration levels significantly and delegated immigrant processing to American embassies in the countries of origin, so Ellis Island became chiefly a deportation and detention center for those who had entered the United States illegally. During World War II it was used to hold enemy aliens—both American citizens of German, Italian, and Japanese descent as well as citizens of those countries who were in the United States at the time war broke out. It was eventually closed entirely in November 1954.
Immigration detention continued even after the closure of Ellis Island. In 1996, Bill Clinton began a policy of mandatory detention that continues to this day. This requires that all unauthorized entrants to the United States, whether they are illegal immigrants, asylum-seekers, or have a different status, must be detained until their situation is regularized or they are deported. Immigrants who are deemed to not be a security risk to the United States may be released on bond until their court date. Others may be detained for weeks if not months. Although the common perception today is that most illegal immigrants are from Mexico, in recent years more have been arriving from the Central American countries of Honduras, Guatemala and El Salvador than Mexico.
In 2018, President Trump decided that the previous policy of releasing families on bond with the promise of appearing in court was too ineffective. He changed this policy, which was derogatorily known as "catch and release" after the fishing program of the same name, to one of mandatory detention for all. Since it is illegal to house children in an adult prison facility, this resulted in children being separated from their parents at the border and being housed in dedicated juvenile facilities.