Troops were set to leave the border at the end of January. Now they're staying through September.

About 2,350 active-duty troops are stationed at the United States' southern border, and they aren't leaving any time soon.

This border patrol mission began in October and was initially scheduled to end on Dec. 15. That date was first extended to Jan. 31, and, on Monday, the Pentagon agreed to a Department of Homeland Security (DHS) request to bump it all the way to the end of September.

Coming months will see some shifts in troops' border responsibilities. The Defense Department "is transitioning its support at the southwestern border from hardening ports of entry to mobile surveillance and detection," said a Pentagon statement, "as well as concertina wire emplacement between ports of entry." Concertina wire is spiraled razor wire that can be installed on stakes as a makeshift fence. Military forces will also continue offering aircraft support for border patrol agents.

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The Posse Comitatus Act prohibits use of the military for domestic law enforcement, and the Trump administration's decision to use U.S. forces at the border has been subject to considerable debate.