Less developed nations frequently use the military to perform law enforcement functions, for three reasons: it centralizes authority in the head of state; the army is frequently viewed as less corrupt than the police; and the army is better at busting heads than the police. What could go wrong?
Plenty. Busting heads doesn’t win you lots of friends in the community, particularly if some of those heads are innocent; the military doesn’t have training in police tactics and may rely on blind force rather than intelligence; the head of state similarly has no particular competence in the field; and the military isn’t necessarily less corrupt than the police. That’s why this tactic rarely works.
Of course, Trump isn’t militarizing America because we are suffering through a crime wave; he simply wants to show people in blue areas that he’s the boss. Think of a dog using his pee to mark a spot as an analogy here.