By Joseph Ax (Reuters) -Hundreds of additional National Guard troops are headed to Washington, D.C., from half a dozen Republican-led states, bolstering President Donald Trump's aggressive move to flood the city with soldiers and federal agents in what he says is an effort to fight violent crime.

The National Guard is a reserve force of the U.S. military that is typically deployed by states in response to natural disasters, although it has been used in the past by presidents to help quell domestic unrest - including President George H.W. Bush during the Los Angeles riots in 1992.