President Donald Trump on Thursday imposed once unthinkably high U.S. taxes on imports from dozens of countries, part of his campaign to turn one of the world’s most open economies into a fortress bristling with barriers to trade.

Full agreements have yet to be reached with the U.S.’s three largest trading partners, Canada, China and Mexico; some Canadian imports have been subject to a 35 percent tariff since early August.

President Donald Trump’s trade war with the world came into force at midnight, with sweeping tariffs hitting scores of America’s commercial partners. But what happens next is profoundly unclear. The two critical questions: Are the tariffs working?