Department of Homeland Security (DHS) Secretary Kirstjen Nielsen resigned from her post Sunday after 16 months in the White House, marking a shift in the Trump Administration’s effort to become “tougher” on border enforcement. President Donald Trump said on Twitter that Nielsen “will be leaving her position, and I would like to thank her for her service.”

Replacing her as the acting head of DHS is U.S. Customs and Border Protection (CBP) Commissioner Kevin McAleenan. Trump added on Twitter, “I have confidence that Kevin will do a great job!”

The shakeup shows the president is set to go in a different direction amid ongoing influx of migrants at the U.S.-Mexico border from Central and South America that has sparked frustration within the White House.

Late last week, Trump withdrew the nomination of acting Immigration and Customs Enforcement (ICE) Director Ron Vitiello to become the permanent head of the agency, explaining that “Ron’s a good man, but we’re going in a tougher direction, we want to go in a tougher direction,” Fox News reports.

Nielsen was seen by administration officials as resistant to some of the harshest immigration measures supported by the president and his aides, including policies involving border enforcement and on other matters like protected status for some refugees.

In her resignation letter posted to Twitter, Nielsen said that “despite our progress in reforming homeland security for a new age, I have determined that it is the right time for me to step aside.”

“We have taken unprecedented action to protect Americans. We have implemented historic efforts to defend our borders, combat illegal immigration, obstruct the inflow of drugs, and uphold our laws and values,” she added.

Speaking to the future of McAleenan’s tenure at the department, Nielsen said, “I hope that the next Secretary will have the support of Congress and the courts in fixing the laws which have impeded our ability to fully secure America’s borders.”