"Mexico must apprehend the remainder or we will be forced to close that section of the Border & call up the Military," Trump tweeted early Wednesday morning.
Trump largely backed off his most recent calls to close the southern border in early April, instead giving Mexico a one-year warning to apprehend more Central American migrants. At the time, Senate Republicans, including Senate Majority Leader Mitch McConnell, publicly warned the president against acting on his threat. Closing the border would have a "potentially catastrophic economic impact," McConnell warned.
While shutting down U.S. border crossings would not stop the vast majority of unauthorized arrivals, who mostly cross illegally between border stations, it would halt the flow of trade between the U.S. and Mexico, which totaled $611 billion in 2018, or $1.67 billion per day. More than 40 percent of all fruits and vegetables brought into the U.S. come from Mexico, for example, according to data from the Department of Agriculture.
The dramatic impact of closing just one port was proven in November, when a port in San Diego was ordered closed for six hours to prevent a large group of migrants attempting to cross from Tijuana. The closure cost $5.3 million in lost revenues, according to the U.S. Chamber of Commerce.
The president's threat comes on the heels of the release of special counsel Robert Mueller's report, which led to calls for Trump's impeachment by some Democrats, including 2020 presidential candidate Elizabeth Warren. Most Democratic leaders of the party have said they won't call for impeachment based on the partially-redacted report and want to first gather more information, including calling on Mueller to testify before Congress.
The president followed with a second tweet about a recent encounter between U.S. and Mexican troops along the border, which CNN first reported last Friday. The president, in his tweet, said Mexican troops "pulled guns" on members of the U.S. National Guard "probably as a diversionary tactic for drug smugglers on the Border," but the situation was more complex.
On April 13, active duty U.S. troops were questioned by five to six Mexican military personnel while conducting border support operations in an unmarked Customs and Border Protection vehicle on the U.S. side of the border near Clint, Texas, U.S. Northern Command (NORTHCOM) said in a statement. The Mexican officials mistakenly believed the Americans to be on the Mexican side of the border and pointed their weapons at the U.S. troops, even removing one soldier's sidearm, according to CNN. The Americans allowed the weapon to be taken in order to de-escalate the situation, which was sorted out after a brief conversation between the two sides.
"Throughout the incident, the U.S. soldiers followed all established procedures and protocols," NORTHCOM said. "An inquiry into the incident is ongoing."
A defense official declined to comment on the Mexican soldiers' intent.
There are currently about 2,900 active duty forces and 2,00 National Guardsmen serving on the southern border in largely a logistical support role to Customs and Border Protection. The troops are not involved in detention operations.
While the president, in his tweet, said the U.S. is "now sending ARMED SOLDIERS to the Border," there is already a combination of armed and unarmed troops supporting the border mission. However, the Pentagon is awaiting a new "request for assistance" from the Department of Homeland Security which a second defense official anticipates will require about 300 to 500 additional U.S. troops.
Acting Defense Secretary Patrick Shanahan earlier this month told reporters that U.S. military support at the border was "elastic" and "it shouldn't come as a surprise that we would provide more support to the border."
ABC News' Luis Martinez, Jordyn Phelps and Quinn Owen contributed to this report.