By calling on troops to suppress protests in Los Angeles on Sunday, Donald Trump has shown he is willing to put the country on a war footing – and test the boundaries of executive power – to achieve his goals. For the first time in decades, the National Guard was deployed against citizens on domestic soil against the wishes of a state’s governor, using a rarely invoked law designed to help the US fight off a foreign invasion. A US president last deployed a state’s National Guard without being asked by its governor in 1965, when Lyndon Johnson sent troops...