President Donald Trump’s controversial military deployment at the southern border will continue until at least September 30 — nine months longer than originally anticipated.

According to a Pentagon statement late Monday night, the Department of Homeland Security requested that troops stay at the border beyond the expected January end date. Acting Defense Secretary Pat Shanahan accepted the support request, and now US service members will transition from “hardening ports of entry to mobile surveillance and detection, as well as concertina wire emplacement between ports of entry,” the statement reads.

It’s unclear how many US troops there are at the US-Mexico border now — near the start of the mission last October, there were roughly 7,000 — but it’s clear their tasks will change slightly.

They will move away from ports of entry — where migrants show up to enter the United States and sometimes claim asylum — and simply move to supporting Border Patrol agents in states like California and Texas.

What hasn’t changed, though, is how limited the military is in what it can do. Armed forces can put up wire on fencing, give US border staff rides on helicopters, and help them surveil the territory. But troops can’t interact with, let alone detain, any people trying to enter America.

The US military is barred from using its capabilities directly to enforce US domestic laws — including immigration laws — unless Congress specifically authorizes it to do so. This measure, known as “posse comitatus,” is why US troops can only support US border agents, but not take direct action themselves.

The extended deployment comes at a politically sensitive time

It’s perhaps not so surprising that America’s armed forces will stay near the US-Mexico border for an extended period of time.

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The government is shuttered over Trump’s desire for about $5 billion in border wall funding, which Democrats refuse to give him. Keeping troops down south, then, may help the Trump administration feel like it is still combating what it calls a “crisis” at the border.

There’s also a new so-called caravan of people moving northward from Honduras. The president expressed worry over the new group last week — similar to how he demonized migrants ahead of the 2018 midterm elections, which led him to send troops to the border in the first place.

“A big new Caravan is heading up to our Southern Border from Honduras,” Trump tweeted on Tuesday morning. “Tell Nancy and Chuck that a drone flying around will not stop them. Only a Wall will work. Only a Wall, or Steel Barrier, will keep our Country safe!”

A big new Caravan is heading up to our Southern Border from Honduras. Tell Nancy and Chuck that a drone flying around will not stop them. Only a Wall will work. Only a Wall, or Steel Barrier, will keep our Country safe! Stop playing political games and end the Shutdown! — Donald J. Trump (@realDonaldTrump) January 15, 2019

Trump continues to fight for that wall. US troops, who probably would want to train to combat America’s enemies instead, will have to do for now.

Dara Lind contributed to this report.