New U.S. Ambassador to Mexico Christopher Landau has arrived in Mexico City to take up one of Washington's most important diplomatic posts, which has been empty for over a year.

Landau replaces Roberta Jacobson, the first woman to be the United States' envoy to Mexico. She resigned and retired in May 2018.

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Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

New U.S. Ambassador to Mexico Christopher Landau has arrived in Mexico City to take up one of Washington's most important diplomatic posts, which has been empty for over a year.

Christopher Landau, the new U.S. ambassador to Mexico, center, walks after delivering a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Landau is a Harvard-educated lawyer who has argued nine cases before the Supreme Court and who clerked for Justices Antonin Scalia and Clarence Thomas in the early 1990s.

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, right, walks after delivering a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, right, walks after delivering a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

The North American neighbors are key commercial partners but Landau's arrival comes after months of tensions over immigration and trade.

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Landau is a Harvard-educated lawyer who has argued nine cases before the Supreme Court and who clerked for Justices Antonin Scalia and Clarence Thomas in the early 1990s.

Landau arrived in Mexico City on Friday morning. He told journalists at the airport that Mexico and the United States face "challenges" just as in all close relationships.

Christopher Landau, the new U.S. ambassador to Mexico, center, walks after delivering a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, center, walks after delivering a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, right, walks after delivering a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, right, walks after delivering a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)

Christopher Landau, the new U.S. ambassador to Mexico, delivers a statement to members of the media at the Benito Juarez International Airport, upon his arrival to Mexico City, Friday, Aug. 16, 2019. (AP PhotoEduardo Verdugo)