The Pentagon is sending about 3,000 additional troops to the southwestern border, rushing to comply with President Trump’s order to increase the military’s role in stemming the flow of migrants into the country.

Armed infantry and support troops from the 4th Infantry Division at Fort Carson in Colorado — one of the Army’s most seasoned combat units — are expected to deploy within days, two Pentagon officials said on Saturday, following Mr. Trump’s declaration on his first day in office that U.S. military forces would confront what he called an “invasion” of migrants, drug cartels and smugglers.

Combined with 1,100 support troops from the military’s Northern Command announced on Friday, and the recently arrived headquarters personnel from the 10th Mountain Division at Fort Drum, N.Y., the reinforcements announced on Saturday will bring the total number of active-duty troops on the border to about 9,000, Defense Department officials said. The Washington Post reported the additional troop mobilization earlier.

This will be the second major wave of active-duty troops sent to secure the border since Mr. Trump took office on Jan. 20. About 1,600 Marines and Army soldiers arrived soon after the inauguration, joining 2,500 Army reservists called to active duty who were already there.

Dispatching large numbers of frontline combat forces indicates that Mr. Trump is breaking with past presidents’ recent practice of limiting deployments along the U.S.-Mexico border mostly to small numbers of active-duty soldiers and reservists.

So far, the active-duty troops have been helping to build barriers and support law-enforcement agencies, as have active-duty and reservist forces sent to the border in past years, including during Mr. Trump’s first term.