As protests spread across the US, President Trump has threatened to send in the army to end the unrest. Mr Trump said he would send in the military if cities and states failed to solve the problem. 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 a 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. "The key point", says Robert Chesney, a University of Texas law professor, "is that it is the president's determination to make; the governors do not have to request his help."