The troops will therefore likely provide reconnaissance, logistics, and assistance to Customs and Border Patrol agents.
Calls for up to 1,000 American troops to be sent to the US-Mexican border have been made by the Department of Homeland Security.
"The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the [U.S. -Mexico-Canada trade agreement]", the president added.
Pentagon spokesman Navy Captain Bill Speaks said the Defense Department was now working with the Department of Homeland Security "to determine the specifics of our support" to border authorities.
Despite Mexican efforts to stop them at the Guatemala-Mexico border, about 5,000 Central American migrants resumed their advance toward the US border Sunday in southern Mexico.
"Every time you see a Caravan, or people illegally coming, or attempting to come, into our Country illegally, think of and blame the Democrats for not giving us the votes to change our pathetic Immigration laws!" he wrote.
Numerous migrants are fleeing poverty and insecurity in Honduras, where powerful street gangs rule their turf with brutal violence.
Migrant Alexander Fernandez said the column planned to move again early on Wednesday.
Many in the caravan may not qualify for asylum even if they reach the border, as the US does not consider escapes from poverty and certain types of violence as qualifying factors.
He is also mobilizing the military along the border, and Secretary of State Mike Pompeo said Tuesday the caravan "will not cross the border under any circumstances". In fact, many immigrants said they'd be just as happy to stay in Mexico indefinitely if they were allowed.
"Go back to your Country and if you want, apply for citizenship like millions of others are doing!"
The Trump administration is planning to dispatch 800 or more active duty troops to the southern border at the direction of a president who has sought to transform fears about immigration into electoral gains in the midterms as a caravan of thousands of migrants makes its way through Mexico.
"It's going to be really frantic", she said.
After spending their second night in the town of Tapachula, the migrants took to the road once more under the sweltering sun to reach Huixtla, a town about 40 kilometers (25 miles) to the north in the Chiapas region.
"If you look back in April of this year, the first caravan that was organized, they came and they played a high stakes game of poker with the United States government", he said.
The first caravan, which has been estimated at 7,000 to 10,000 mostly Honduran migrants fleeing violence and poverty in their homelands, is now in the town of Huixtla in Chiapas state around 50 km (31 miles) north of the Guatemalan border.