The president made the claims five days before the US midterm elections, when voters will determine which party will hold power in Congress and statehouses across the country.
Trump made the announcement after a wave of falsehoods, many of them racially charged, about what he again called an "invasion" of Central American migrants and asylum seekers now traveling toward the USA border. Immigration officials have turned away asylum-seekers at border crossings because of overcrowding, telling them to return at a later date.
The speech follows a series of proposals and statements on immigration that Trump's opponents have called outlandish stunts.
But immigrant advocates contend the government is violating the U.S. Immigration and Nationality Act, which says any foreigner who reaches the U.S. has the right to apply for asylum. "We have a lot of tents, we have a lot of everything, we're going to hold them right there", he said.
Trump and other administration officials have long said those seeking asylum should come through legal ports of entry. He said he told the armed troops deployed to the border that they could open fire if migrants throw rocks at them, something Border Patrol agents have done in the past. What they did to the Mexican military is a disgrace.
Trump also tweeted a video Wednesday alleging Democrats were responsible for allowing a homicidal immigrant into the USA but provided no evidence supporting that claim.
The memo, obtained by Newsweek, states that troops "may perform missions that require direct contact with migrants and/or the public", which "may require them to be armed". "You put them and all law enforcement in a very hard emotional, let alone security posture".
U.S. President Donald Trump speaks to reporters before departing on a campaign trip on the South Lawn of the White House in Washington, U.S., November 2, 2018. Although the President referenced such a policy in his speech, he offered no defense of how such a plan, once finalized, could be legal, given laws presently allowing migrants the right to claim asylum once they are on American soil. "I always want to tell the truth, when I can I tell the truth", Trump told ABC News in an interview. He claimed, without providing proof, that former President Barack Obama's administration separated children from the parents but "nobody talks about that". Trump, for several months, systematically separated every family apprehended at the border.
Thousands of Central American migrants resumed their slow trek through southern Mexico yesterday as immigration agents and police nibbled at the edges of the two caravans now in the country.
Khashoggi's body cut into pieces and 'dissolved'
His killing has caused countries worldwide to re-examine their relationship with the kingdom, especially the United States. Iran has denied accusations it is building a nuclear bomb, saying weapons of mass destruction are prohibited under Islam.
Mr Trump has hardened his stance on migration in a bid to rouse his political base ahead of congressional elections next week.
The migrants also were also turned off by the USA president's characterization of the slow-moving caravan and the three smaller ones following as "invasion".
"It's a classic strategy that goes back to 19th-century nativist thinking", he said.
Those caught at the border will be held in tent camps or other facilities until they can be deported or have their requests approved, he said. "Vote Republican now!" The ad claimed, "President Donald J. Trump and Republicans are making America safe again!" "They do that with us they're going to be arrested for a very long time". The migrants are still more than 1,000 kilometers (more than 620 miles) from the USA, a distance that will likely take several weeks for them to walk.
"Get off! Get off!" police officer Benjamin Grajeda shouted to a group of migrants clinging to the side of a truck outside Juchitan. "Opioids are a bigger threat". Just a week ago the caravan was reportedly as large as 7,000 people but Fox News reports that number had dropped down to about 4,000.
In a meandering speech at the White House about immigration, Trump recycled numerous talking points he touts on the campaign trail - but offered little in the way of concrete ways to address the problems he embellished.
"Asylum was never meant to alleviate all problems, even all serious problems, that people face every day all over the world", Sessions said in June.
Asylum is not a program for those living in poverty. "The United States can not possibly absorb them all", Trump said during a news conference at the White House.