President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE signed a proclamation late Wednesday ordering National Guard troops be sent to the U.S.-Mexico border to address a “surge of illegal activity.”

In a memo to Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE and Homeland Security Secretary Kirstjen Nielsen Kirstjen Michele NielsenDHS IG won't investigate after watchdog said Wolf, Cuccinelli appointments violated law Appeals court sides with Trump over drawdown of immigrant protections Democrats smell blood with new DHS whistleblower complaint MORE, Trump ordered the National Guard be used to secure the border “to stop the flow of deadly drugs and other contraband, gang members and other criminals, and illegal aliens into this country.”

Trump also asked Mattis and Nielsen to submit reports within 30 days detailing what other resources are needed at the border.

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Trump first suggested on Tuesday that he'd like to deploy troops to the southern border to secure the area until his proposed wall can be built.

On Wednesday, he followed through on that idea. Nielsen spoke at the White House press briefing to explain the deployment of troops.

“It’s time to act,” Nielsen told reporters, adding that the deployments could begin “immediately.”

She did not share how many troops will be sent to the border, the length of the deployment or its cost.

Trump has spent recent days tweeting his frustrations about current immigration laws, warning of "caravans" of migrants approaching the border and calling on Congress to enact tougher regulations.