This week, members of the caravan were making their way through a border crossing in southern Mexico, crossing from Guatemala into Ciudad Hidalgo in the state of Chiapas near the Pacific coast - roughly 1,100 miles from the USA border, via the most direct route to Brownsville, Texas.
Trump added on Thursday, Oct. 25, "I am bringing out the military for this national emergency".
The additional troops are to provide what one official described as logistical support to the Border Patrol. There have been instances where these situations have occurred in the past and this was merely a proactive effort to inform stakeholders with property along the immediate border.
The troops are part of a security operation that is expected to be up and running by October 30.
Many migrants in the sprawling caravan - once estimated by the United Nations to number more than 7,000 - are hoping to make it to the United States.
There are already about 2,000 National Guard troops assisting at the border under a previous Pentagon arrangement. Most are Hondurans, seeking to escape poverty and violence, and include families with children.
The president first threatened to send troops to the U.S. -Mexico border in April, telling foreign leaders that America would be "guarding our border with our military".
In a tweet sent on Thursday, President Trump addressed the migrant caravan directly, advising them to follow the laws of the United States and apply to come in legally.
"They will be stopped!"
"This caravan can not come to the United States", Nielsen said. "Go back to your Country and if you want, apply for citizenship like millions of others are doing!" tweeted Trump.
Several administration officials say it is anticipated that the Pentagon will pay for the bulk of the mission by using active duty troops.
The ones from Texas aren't armed and are helping in key support roles such as running motor pools so more Customs and Border Protection agents can work directly along the border, said Texas Army National Guardsmen spokesman Maj.
The troops will also provide tents and medical care for border authorities in those areas.
"Wait'll you see what happens over the next couple of weeks".