President Donald Trump’s order to deploy U.S. troops to the border with Mexico isn’t the first time they’ve been there in the modern era.

Excluding the Mexican-American War that ended in 1848, modern presidents have deployed U.S. troops on the southern border for the war on drugs, to slow illegal immigration and as a safety measure after the 9/11 terrorist attacks.

Trump’s April 4 order calls on Defense Secretary Jim Mattis to support the Department of Homeland Security. “The Secretary of Defense shall support the Department of Homeland Security in securing the southern border and taking other necessary actions to stop the flow of deadly drugs and other contraband, gang members and other criminals, and illegal aliens into this country,” Trump said, asking for National Guard deployments. That does not rule out the possibility of Regular Army deployments in support of the border mission.

The action is needed, Trump said in the memo, because illegal activity on the U.S.-Mexico border “has now reached a point of crisis.”

While there isn’t a fixed number, Trump told reporters he is looking to send “anywhere from 2,000 to 4,000” troops who are likely to be National Guard forces mobilized under state authority. The duration of the border mission also isn’t fixed, he said. “We’ll probably keep them, or a large portion of them, until such time as we get the wall,” he said, referring to a 1,000-mile wall between the U.S. and Mexico he promised to build during his presidential campaign. Securing funding for the wall has proven difficult, with the president suggesting he might tap the Pentagon budget to pay for at least part of the structure.

The move follows similar efforts by previous commanders in chief who sent troops to the U.S.-Mexico border on similar grounds.

President George H.W. Bush deployed troops to the border in 1989 on drug surveillance and interdiction missions in support of law enforcement. In 2010, President Barack Obama sent 1,200 troops to the border and President George W. Bush deployed 6,000 in 2006.

U.S. service members have aided law enforcement agencies and the Department of Homeland Security with intelligence-gathering, communications and surveillance capabilities, and have also provided logistics and transportation support. There is a formal process with Joint Task Force-North for federal law enforcement agencies to request intelligence support from regional military teams in California, Arizona, New Mexico, Texas and the Caribbean.