They've trained for years to do guerrilla hand-to-hand urban combat. Probably never thought they'd have to use that training right here in the U.S.
Nothing worse than civil war. No one wants to go up against a fellow citizen.
Btw, Mattis is one of the experts on insurgency and counter-insurgency. Here's the thing he wrote with Patreus:
file:///D:/2018DSM2019&2020/Q+/PDF%20Citations/MCWP%203-33.5_Part1MattisPatreus.pdf