EXPLAINER-What is the Alien Enemies Act of 1798 that Trump wants to use in deportations?

March 17 (Reuters) - U.S. President Donald Trump invoked the Alien Enemies Act in a proclamation made public on Saturday as part of his pledge to deport millions of people who are in the country illegally.

Below is a look at the act and how it has been used in the past.

WHAT IS THE ACT?

The Alien Enemies Act was enacted in 1798 to combat spying and sabotage during tensions with France. It authorizes the president to deport, detain or place restrictions on individuals whose primary allegiance is to a foreign power and who might pose a national security risk in wartime.

The act states it can be invoked "whenever there is a declared war" or "any invasion or predatory incursion" that has been perpetrated, attempted or threatened against the United States by a foreign government.

The act requires the president to publicly proclaim the event that prompted the act to be invoked. The act remains in effect until the president terminates it.

HOW HAS THE ACT BEEN INVOKED?

The law was used in the War of 1812 between the United States and Britain and in both World Wars and was used to detain and deport individuals, as well as restrict their freedom.

President Woodrow Wilson used the act to bar citizens of enemies of the United States from possessing firearms and explosives, residing in certain areas and publishing certain materials, among other restrictions.

President Franklin Roosevelt used the act to justify internment camps for people of Japanese, German and Italian descent during World War Two. President Harry Truman continued to use the act until 1951, after hostilities had ceased in World War Two, according to the Brennan Center for Justice.

HOW DID TRUMP JUSTIFY INVOKING THE ACT

Trump said the Venezuelan gang Tren de Aragua, which the U.S. government has declared is a foreign terrorist organization, was "conducting irregular warfare and undertaking hostile actions against the United States" with the goal of destabilizing the nation.

He said in the proclamation that the group, which has been linked to kidnapping, extortion, organized crime and contract killings, infiltrated Venezuela's government.

WHAT HAVE COURTS SAID ABOUT USE OF THE ACT?

Individuals have sued to challenge their detention or removal, but most of the cases have turned on questions of the person's citizenship.

The act has been upheld as constitutional and the Supreme Court has said it can even be used after wartime.

In 1948, the Supreme Court ruled the government could deport Kurt Ludecke, a former Nazi who fell out with the party, escaped a concentration camp and came to the United States, even though the war with Germany was over. The court said it would have been impractical to deport him while the war was going on.

Some Democratic lawmakers in the U.S. House and Senate reintroduced a bill in January that would repeal the Alien Enemies Act, pointing to its use in the internment of Americans and arguing it violates civil and individual rights.

WHO DECIDES IF THE U.S. HAS BEEN INVADED?

In addition to a declared war, the act can be invoked when there is an "invasion" or "predatory incursion."

Courts have been asked what constitutes an invasion, although in cases unrelated to the Alien Enemies Act.

California brought a lawsuit against the federal government in the 1990s claiming it was failing to protect the state from an invasion of individuals crossing the southern border illegally.

The court decided that determining what constitutes an invasion was a political question for the other branches of government. The court also said that there was no manageable standard for determining when an influx of individuals rose to the level of an invasion.

Courts have also said an influx of individuals entering the country illegally was likely not considered an invasion based on the writings of the founding fathers, who understood the term to mean armed hostility by another state or foreign country.