This is just the beginning,” one Defense official said about the deployment of active-duty troops to the border with Mexico.

President Donald Trump’s promised crackdown on illegal immigration has begun to take shape in San Diego County, as federal agents launched targeted operations to arrest undocumented immigrants and a wave of military troops got to work reinforcing U.

President Trump has deployed 1,500 active-duty troops to the U.S. border with Mexico. But a nearly 150-year-old law, the Posse Comitatus Act, bars using the military for domestic law enforcement purposes.