As protests spread across the US, President Trump has threatened to send in the army to end the unrest.
“If a city or a state refuses to take the actions that are necessary to defend the life and property of their residents, then I will deploy the US military,” Mr Trump has said.
But some state governors have said the government does not have the power to send in federal troops without the permission of the state authorities.
Can the president deploy troops?
In short, yes under certain circumstances.
There are already thousands of troops deployed from the National Guard, which is the reserve force for the US Army.
They’re in more than 20 states across the US trying to quell protests, but these forces have been requested by the cities or states themselves.
However, a US law passed in the 19th Century lays out circumstances when the government in Washington DC can intervene without state authorisation.
The Insurrection Act says the approval of governors isn’t required when the president determines the situation in a state makes it impossible to enforce US laws, or when citizens’ rights are threatened.
The law was passed in 1807 to allow the president to call out a militia to protect against “hostile incursions of the Indians” – and it was subsequently extended to allow for the use of the US military in domestic disturbances and to protect civil rights.
Another law passed in 1878 requires congressional authorisation for domestic military use, but a legal expert told the BBC that the Insurrection Act was sufficient legal authority on its own for the president to deploy the army.
It is widely accepted that the president would have legal grounds to employ the military without asking for approval from the states in the current circumstances.
Stephen Vladeck, a University of Texas law professor, has been quoted in the US media as saying: “Historically and practically, such a request is not necessarily a prerequisite to the president using regular federal troops for domestic law enforcement.”
Has the law been used before?
According to the Congressional Research Service, the Insurrection Act has been used dozens of times in the past, although not for almost three decades.
It was last invoked in 1992 by former US President George HW Bush during race riots in Los Angeles.
The law was used throughout the 1950s and 60s during the civil rights era by three different presidents, including when there were objections from state governors.
President Dwight Eisenhower faced objections when he used the law in 1957 to send US troops to Arkansas to control a protest at a school, where black and white children were being taught together.
Since the end of the 1960s, the use of the law has been rare. Congress amended it in 2006 following Hurricane Katrina in an attempt to make military assistance more effective, but the amendment was repealed after state governors objected to it.