President Donald Trump will have few options to deal with the thousands of migrants from Central American countries — along with countries in the Middle East, Asia, and elsewhere — if the caravan now in southern Mexico reaches and crosses the U.S. border.
Trump sought to persuade Mexican officials to stop the caravan for the past two weeks, as it has steadily grown after launching from Honduras, by threatening to reduce or end U.S. aid.
He is also mobilizing the military  along the border, and Secretary of State Mike Pompeo said Tuesday the caravan “will not cross the border under any circumstances.” The problem comes if individuals or groups of individuals within the caravan do get across the border into the U.S. Then, Trump’s options become much more limited.
Under current immigration law, for example, all individuals are entitled to certain rights and legal protections while in the custody of the U.S. government, including illegal aliens.
An illegal alien can claim asylum whether they crossed at a legal port of entry or illegally at any other point along the border. Immigration and Customs Enforcement (ICE) then places them into proceedings  to determine if their asylum claim is merited.
“Once they’ve been granted an interview, they are in the U.S. system and they crossed the border legally, they haven’t tried to enter illegally, then the options get a lot smaller,” Center for Strategic and International Studies senior fellow Richard Miles told LifeZette.
“Unless Congress decides to change the law, the administration has to follow the procedure, and my guess is the administration is not going to want to have another political disaster on its hands like separating families,” Miles said.
The illegal alien will then have the chance to make his or her case before an immigration court. The immigrant can contest any charges filed, usually unlawful entry, or claim asylum. But that process can take years, and many immigrants are even allowed to place bail. It becomes much more complicated if they have children with them.
Officials at ICE and the immigration courts are already dealing with a massive backlog of cases , which the caravan could make much worse. There were over 679,000 immigration cases pending in immigration courts, more than three times the total in 2009. The cases still pending before immigration courts have already lasted an average of 722 days without resolution.
ICE is the main agency for enforcing immigration law, but they can get support from local law enforcement and other federal agencies. The Immigration and Nationality Act gives the attorney general the power to authorize  state and local law enforcement officers to assist federal immigration officials, but they would be bound by the same restrictions. Local police and state law enforcement officials customarily cooperate with federal officials, except in sanctuary cities.
Trump’s deployment of the military along the border would also face restrictions that make it more difficult to deport illegal aliens. The Posse Comitatus Act forbids the military from enforcing civilian law , which puts them more in a support role.
Trump isn’t the first president to deploy the military along the border, as both of his immediate predecessors, former Presidents George W. Bush and Barack Obama, did so. The General Accountability Office (GAO) looked at those deployments and concluded government officials did not understand  what the military was allowed to do on the border.
Trump could also issue an executive order that blocks anyone involved in the caravan from entering the country. Federal immigration laws allow the president to suspend the entry  of all aliens or any class of aliens he feels would be detrimental to the interests of the United States. But such an action could be tied up in court for years, as happened with his travel ban.
The Supreme Court eventually upheld the president’s authority to issue such executive orders blocking entry into the U.S. by individuals from specific countries. But this only works as a tool to prevent entry and would have no effect on individuals already in the country illegally.