O'Shaughnessy said the mission of what's being called Operation Faithful Patriot is to harden the border in preparation for the caravan of asylum-seeking migrant families making its way north through Mexico.
He added: "This is an invasion of our Country and our Military is waiting for you!"
The announcement during a Fox News interview came as the Pentagon announced it was deploying 5,200 active-duty troops to beef up security, and follows weeks of heated anti-migrant rhetoric from the president ahead of crucial midterm congressional elections next week.
Three U.S. officials say the number of military troops deployed to the southern border in support of the Customs and Border Patrol could be in the thousands.
The number of people in the first migrant caravan headed toward the USA has dwindled to about 4,000 from about 7,000 last week, though a second one was gaining steam and marked by violence.
CBP Assistant Commissioner of Public Affairs Andrew Meehan said in a news conference Monday afternoon that there are approximately 3,500 people in one group, which is now at the Chiapas-Oaxaca border in southern Mexico.
Trump said the United States would build "tent cities" to house migrants seeking asylum, rather than releasing them while they await court decisions. Guatemalan firefighters confirmed that a 26-year-old Honduran was killed from a rubber bullet hitting his head.At a news conference late Sunday, Mexican Interior Secretary Alfonso Navarrete Prida denied that his country's forces were responsible.He said that Mexican federal police and immigration agents were attacked with rocks, glass bottles and fireworks when migrants broke through a gate on the Mexican side of the border, but that none of the officers were armed with firearms or anything that could fire rubber bullets. The National Guard is often used by states to help with border security.
Air Force General Terrence O'Shaughnessy, head of the United States military's Northern Command, told reporters the 5,200 troops would focus on trying to "harden" border crossings and surrounding areas, with work done by combat engineering battalions with experience building temporary fencing.
The president has stepped up his focus on immigration in the days leading up to the midterm elections.
The Pentagon already has sent 22 miles of concertina wire to the 1,954-mile-long border, he said, and has enough additional wire to cover 150 miles. The troops expected to be sent to the border will primarily consist of military police and engineers, but the military also reportedly plans to send Marines. But active duty troops are rarely deployed within the United States except for domestic emergencies like hurricanes or floods.
He continued his threats on Monday, tweeting, without providing evidence, that, "Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border".
The U.S. military has already begun delivering jersey barriers to the southern border in conjunction with the deployment plans.