U.S. President Donald Trump threatened to impose a 50-per-cent tariff on more than 500 Canadian goods effective next month. This is far from the first time the Trump administration has threatened levies on Canada. But this time, the administration has pulled a lever that dates all the way back to the 1930s.Today, Adrian Morrow, The Globe’s U.S. correspondent based in Washington, walks us through the Smoot-Hawley Act Trump has invoked, the strategy behind using it and what legal challenges the law could face.