Long stretches of the border are under the supervision of nearby military bases, empowering U.S. troops to detain people entering the country illegally.
More than 6,000 troops are poised to march near the National Mall to honor the Army’s 250th anniversary on Saturday, which happens to be Trump's birthday.
Trump's partisan comments received cheers or jeers from the crowd, an expression of political views in uniform that would typically result in a reprimand.
The Friday night discussion on veterans issues is part of a series of events from Democrats scheduled around the controversial Army parade in Washington.