Thousands of active-duty US troops are immediately deploying to the US-Mexico frontier to tighten security and provide back up to overstretched border agents.
Critics have blasted the move as an expensive political stunt ahead of next week's midterm elections.
Here is a look at what the deployment means, and what American soldiers will be doing on the border.
US authorities say troops are needed to address large groups of migrants arriving at the border in so-called caravans, made up largely of Central American migrants.
The issue has incensed President Donald Trump, who has sought to make political capital by depicting the migrants as carrying out an "invasion" of America.
He has repeatedly hammered on the caravans and tried to keep the issue in the headlines ahead of crucial midterm congressional elections that could see the Democrats regain some degree of power.
The Democrats, for their part, have seemed to founder in terms of presenting a coherent message on border and immigration issues and the caravans.
Numbers are changing daily, but so far the Pentagon has authorized 5,239 active-duty troops to go to the border on a 45-day deployment.
Another 2,000 or more are on standby, and some 2,100 National Guardsmen are already in the border region to support agents there.
That means more than 9,000 troops have already been committed to military operations on the border and Trump, who is prone to hyperbole, on Wednesday said the final number could swell to 15,000 troops.
Whatever the case, it's a massive deployment, the largest since Defense Secretary James Mattis came into office nearly two years ago.
It means that within days, the US military will have more than three times as many troops along the southern border as it does fighting the Islamic State group in Syria.
Approximately 15,000 US troops are in Afghanistan.
The soldiers -- who will be armed -- taking part in "Operation Faithful Patriot" are not meant to directly interact with migrants coming into the US. Rather, they will provide logistical support to agents with the US Customs and Border Protection Agency.
General Terrence O'Shaughnessy, head of the US military's Northern Command, said troops will focus on trying to "harden" border crossings and surrounding areas, with work done by combat engineering battalions with experience building temporary fencing.
The soldiers will also provide medical support and temporary housing for border guards, and ferry them around on military helicopters.
A first caravan, which has dwindled and now numbers some 4,000 people, according to an aid group called Pueblos sin Fronteras, arrived Tuesday in the town of Juchitan, in Oaxaca.
A smaller group of 2,000 people managed to cross the Suchiate River separating Mexico from Guatemala the day before.
The migrants are moving slowly, mostly by foot, and are still hundreds of miles from the US border.
They might not even arrive there until after the 45-day deployment is over.
Mattis on Wednesday rejected criticism that the deployment is a political move ahead of next week's elections.
"We don't do stunts in this department," he said.
But critics have piled on and said Trump does not have the legal authority to order the military to enforce domestic laws.
"This deployment -- which comes at the expense of millions of taxpayer dollars, and an uncertain impact on our military readiness -- might satisfy the president's ego but not much else, given all the restrictions on what the military can legally do in support of our law enforcement authorities at the border," Democratic Senator Patrick Leahy said in a statement.
The Pentagon has not provided an estimate of costs of the operation.
Republican Congressman Jason Smith said he was glad Trump is taking a stance on the caravan issue.
"Our laws and sovereignty have to be respected," he wrote on Twitter.