Roberta Jacobson, the U.S. ambassador to Mexico, has handed in her resignation. The career diplomat, with more than 30 years in government service says it was a difficult decision to leave. Jacobson, 57, is the latest in a string of high-level diplomats to depart the State Department since President Trump took office.

In a note to embassy staff, Jacobson said, "The 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."

She said her resignation will be effective May 5.

Jacobson, who was appointed by President Obama and assumed her post in 2016, did not give a reason for her resignation. But according to former U.S. and Mexican diplomats, the strain in the countries' relations made her job particularly difficult.

Trump's disparaging comments about Mexicans and immigrants has incensed many in Mexico, as have his continued calls for Mexico to pay for a U.S. border wall. A tense telephone conversation last month between Trump and Mexico's President Enrique Peña Nieto soured over the wall issue, and derailed chances of an in-person meeting between the two leaders.

In addition, negotiators are currently in Mexico City, locked in the seventh round of contentious talks to renegotiate the North American Free Trade Agreement. Trump has frequently threatened to walk away from the trade pact if the U.S. does not get what he considers a better deal.

"This has been a very challenging time and my respect for Roberta [Jacobson] has been extremely high, she has had to maneuver this with a great deal of skill," said one former U.S. diplomat who only agreed to speak on background.

Jacobson has had a long career in government service, most of it focused on Latin America, where she served as Assistant Secretary of State for Western Hemisphere Affairs, before coming to Mexico as ambassador. She is well-known and well-liked in Mexico.

"This is a huge loss, for both countries," says Arturo Sarukhan, a former Mexican ambassador to the U.S. who has known and worked with Jacobson since the 1980s.

Sarukhan says he hasn't seen U.S.-Mexico relations this low in decades. "She is a stellar government official ... she was a great partner ... and she will be sorely missed and I think the relationship losses her at a trying moment," he says.

Jacobson's departure comes on the heels of several veteran career diplomats leaving the State Department, including many with deep experience in Latin America. Several departing officials have stated they can no longer work in the Trump administration, one saying that the Secretary of State and the President have abandoned human rights around the world and disregarded diplomacy. In January, the U.S. Ambassador to Panama announced his resignation saying he no longer could advocate for Trump.

Mexico's Foreign Service says it was informed of Jacobson's resignation on February 17 and is ready to accept her replacement. The State Department has not announced Jacobson's successor. The Mexican newspaper Reforma and several U.S. news outlets, citing sources, say the new ambassador will be Ed Whitacre, a former AT&T president from Texas.

Whitacre has also been a board member of Exxon Mobil and a former president of the Boy Scouts. Secretary of State Rex Tillerson, the former CEO of Exxon Mobil, was also a Boy Scout past president.

Experts on U.S.-Mexico relations say they hope the Trump administration will move quickly to fill the ambassadorship. Leaving it vacant for a long period of time is not good for the relationship already under such strain, says Eric Olson, an advisor to the Mexico Institute at the Wilson Center in Washington, D.C.

"How quickly they move to replace Roberta Jacobson in Mexico is going to be really, really important because we can't have this kind of relationship running on auto-pilot," says Olson.

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