Investors exhaled last weekend when Donald Trump pulled back from an escalatory tariff war with Mexico. But their relief is bound to be shortlived. Whenever the US president finds fault with another country, he reaches first for tariffs. His grievances need have nothing to do with trade: in Mexico’s case, Mr Trump was furious at the persistence of the migrant crisis on the US border. Nor does he try to link his actions to statutory cause. Mr Trump threatened Mexico under a 1977 act that authorises the president to impose penalties on America’s enemies, terrorist states and other pariahs. Mexico does not resemble any such country.

The same applies to his serial misuse of section 232, intended to protect the US from strategic threats, to impose tariffs on America’s friends. Under no sensible assessment can Canadian metals, or European cars, be defined as a threat to US national security. Mr Trump shows every sign of continuing to abuse his powers. It is long overdue for Congress to reclaim its constitutional role on trade.

To be fair to the normally acquiescent Republicans, Mr Trump’s party has led the way in opposing his Mexico belligerence. Normally loyal senators, such as Ted Cruz of Texas and Iowa’s Chuck Grassley, attacked Mr Trump’s over-reach. Others, including Utah’s Mike Lee and Pennsylvania’s Pat Toomey, are drafting a bill that would limit the president’s powers. Such a law is firmly in the US national interest.