Under the Posse Comitatus Act, the U.S. military is prohibited from enforcing laws on domestic soil without explicit constitutional or congressional authorization. Given that the Insurrection Act has not been invoked, the fact that Marines have detained a civilian on U.S. soil for the first known time in the country's history represents an unprecedented authoritarian overreach by the Trump administration.
It's no surprise that Democrats are overreacting — this is what they do when Trump is in office. Marines merely detained an individual that crossed a yellow-tape boundary around the Wilshire Federal Building, and even the man involved is fine with the procedure. Claims of authoritarian overreach by the Trump administration reflect political bias rather than the actual events.