Let’s start with the basics. First, there is no credible basis for the president’s assertion that the migrants are dangerous. They are hundreds of miles away from the U.S. border and are the very definition of Emma Lazarus’s tired, poor and huddled masses. They should not be blocked by bayonets but met by a joint U.S.-Mexico operation that provides humanitarian support and processes them for possible admission to the United States in accordance with immigration and asylum laws.

Second, even if any of the migrants prove to be dangerous or cross the border illegally, it does not follow that this is a job for the U.S. military. Federal law-enforcement agencies — for instance, the Border Patrol — are surely up to the task. But more to the point, the Posse Comitatus Act of 1878 prohibits the use of military personnel to enforce U.S. laws on American soil. To bypass Posse Comitatus, the Trump administration must establish that the migrants are a threat to national security. If the administration succeeds, it will set a precedent for the use of military force domestically to combat organized crime syndicates, street gangs or, perhaps, white supremacist groups — all of which strike us as more violent and more threatening to national security and the rule of law. But combating them, too, is a matter for the police and prosecutors, not the military.

Third, Trump is risking the politicization of the armed forces. Enforcement of U.S. immigration laws is important, and no responsible person advocates open borders. But the president appears to have cynically chosen to exploit irrational fear of nonwhite immigrants for political ends, using the military in the service of that goal.