President Trump has secretly signed a directive to the Pentagon to begin using military force against certain Latin American drug cartels that his administration has deemed terrorist organizations, according to people familiar with the matter.

The decision to bring the American military into the fight is the most aggressive step so far in the administration’s escalating campaign against the cartels. It signals Mr. Trump’s continued willingness to use military forces to carry out what has primarily been considered a law enforcement responsibility to curb the flow of fentanyl and other illegal drugs.

But directing the military to crack down on the illicit trade also raises legal issues, including whether it would count as “murder” if U.S. forces acting outside of a congressionally authorized armed conflict were to kill civilians — even criminal suspects — who pose no imminent threat.