United States’ ambassador to Mexico Roberta Jacobson announced her resignation Thursday, in a surprise decision likely to add more pressure to an already strained U.S.-Mexico relationship.

Jacobson’s announcement comes days after Mexican President Enrique Peña Nieto canceled a visit to the U.S. following a heated phone call with President Donald Trump. Her departure is the latest in a series of high-profile exits from the State Department, including numerous diplomats and seasoned officials, during Trump’s tenure.

“I have come to the difficult decision that it is the right time to move on to new challenges and adventures,” Jacobson said in her resignation letter, as reported by the New York Times. “This decision is all the more difficult because of my profound belief in the importance of the U.S.-Mexico relationship and knowledge that it is at a crucial moment.”

Jacobson will be resigning in May after serving just two years as ambassador. She was nominated by President Obama in 2015, but a partisan fight led by Marco Rubio based on her work to warm relations with Cuba kept her from being confirmed for nearly a year.

She spent most of her 31-year Foreign Service career in Latin America, including as the Deputy Assistant Secretary for Canada, Mexico and NAFTA issues from 2007 to 2010, and as the director of the Office of Mexican Affairs from December 2002 to 2007.

Trump has had a tense relationship with Mexico from his first day on the job, which probably didn’t make Jacobson’s life any easier. Trump kicked off his campaign in June 2015 by saying of Mexican immigrants, “They’re bringing drugs. They’re bringing crime. They’re rapists. And some, I assume, are good people.”

He has also famously insisted that Mexico is going to pay for a wall between the two countries. To which former Mexican President Vicente Fox famously said, “Mexico is not going to pay for that fucking wall.” And Trump hasn't really “evolved” on the wall issue, despite White House Chief of Staff John Kelly's suggestions to the contrary.

Trump has also repeatedly railed against NAFTA, the trade deal between the U.S., Mexico, and Canada that’s been in place since the Clinton administration. He’s called it the “worst deal in history.”