U.S. Defense Secretary Jim Mattis and Homeland Security Secretary Kirstjen Nielsen visited the U.S. southern border Wednesday, where thousands of American troops have been deployed to support border patrol agents.Speaking to reporters en route to McAllen, Texas, Mattis stressed that “border security is part of national security.”“It’s obviously a moral and ethical mission to support our border patrolmen,” the defense secretary said. “Our units are in a position to enable the border patrol’s law enforcement operations.” President Donald Trump called for increased border security to prevent a caravan of asylum-seeking migrants from Central America from entering the United States, declaring on several occasions the caravan's approach constituted a "national emergency." More than 5,600 active duty U.S. troops and another 2,100 National Guard troops have moved into the U.S.-Mexico border area in response to requests for help from the Department of Homeland Security. The military personnel assigned to help with border security have been helping with barriers, fencing and aerial support. They are legally prohibited from engaging in domestic law enforcement, such as arresting migrants crossing the border.Mattis told reporters Wednesday the U.S. military has a long history of border deployments. He highlighted 1916, when President Woodrow Wilson deployed troops to stop the threat of Pancho Villa’s troops during the Mexican Revolution, and 1995, when National Guard and several hundred active duty personnel were assigned to assist law enforcement.Mattis did not mention a deployment in 1997, when an active duty Marine corporal shot a Texas teenager during an anti-drug operation along the Texas-Mexico border, causing the Pentagon to suspend its anti-drug operations there. Mattis and Nielsen’s border visit comes as sources say Trump is considering removing Nielsen from her post. Nielsen was selected as homeland security secretary after John Kelly left the position to become Trump's chief of staff.