On April 11, President Donald Trump announced his intention to turn a strip of federal land along the U.S. southern border into a massive "National Defense Area," effectively creating a new 170-square-mile U.S. military installation. The plan was described in a National Security Presidential Memorandum (NSPM-4), titled "Military Mission for Sealing the Southern Border of the United States and Repelling Invasions." The document sets in motion the creation of a military area that is twice the size of Washington, D.C. It also expands the U.S. military's role in stopping migrants crossing the border.