Department of Homeland Security Secretary Kirstjen Nielsen has resigned after 16 months on the job and immense pressure from the White House and public over the situation along the U.S. southern border.

“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,” Nielsen wrote in her resignation letter to President Donald Trump.

While she did not say exactly why she is quitting, Nielsen wrote she hopes 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 and which have contributed to discord in our nation’s discourse. Our country and the men and women of DHS deserve to have all the tools and resources they need to execute the mission entrusted to them.”

Analysts have said Nielsen was frustrated by what she saw as a lack of cooperation from Congress and the courts in tackling illegal immigration.

Trump has also expressed frustration with the situation along the U.S. border with Mexico, where hundreds of thousands of migrants trying to escape poverty and crime in central America hope to enter the U.S.

White House sources have said Trump often yelled at Nielsen for apparently not being strong enough in curbing the flow of migrants trying to enter the U.S. They say she had to listen to what those sources called Trump’s impossible demands.

Along with the pressure from the White House to try to stop the influx of migrants, Nielsen faced a public outcry over the administration’s highly unpopular policy of separating migrant families when they crossed into the U.S.

Thousands of young children were taken from their parents and held in separation facilities, often in less than ideal conditions. Nielsen was responsible for executing that policy while at times denying there was such a policy.

Despite the acrimony, Nielsen wrote, “I can say with confidence our homeland is safer today than when I joined the administration. We have taken unprecedented action to protect Americans.”

President Trump tweeted Sunday that “Nielsen is leaving her position and I would like to thank her for her service.”

He announced that current U.S. Customs and Border Protection Commissioner Kevin McAleenan will become acting DHS secretary.

“I have confidence that Kevin will do a great job,” he tweeted.
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