On March 1 Donald Trump announced that his Administration would implement a 25% tariff on all steel imports and a 10% duty on aluminum. President Trump initially said the tariffs would apply to all steel and aluminum exporters, but a week later, facing pressure from Republican free-traders, temporarily excluded Mexico and Canada while NAFTA talks continue.
The tariffs were needed for national security reasons, argued Trump, based upon an investigation by the US Commerce Department which found in January that cheap foreign steel was indeed a threat to America:
The continued rising levels of imports of foreign steel threaten to impair the national security by placing the U.S. steel industry at substantial risk of displacing the basic oxygen furnace and other steelmaking capacity, and the related supply chain needed to produce steel for critical infrastructure and national defense. - US Commerce Department
The announcement caused an immediate drop in stock markets and a chorus of outcries from US trading partners, who said they would impose retaliatory tariffs on American exports and fight the trade restrictions at the World Trade Organization.
But they shouldn't have been surprised. During the election campaign Trump blasted China for a host of American economic ills including an artificially low exchange rate, dumping steel into the US market, and funnelling billions of dollars in subsidies to its steel mills.
Chinese steel companies, which produce half of the world's steel, have long been a target of US import duties, and the 25% tariff was clearly aimed at them. There are currently 169 antidumping and countervailing duty orders on steel, with 29 of them against China.
The largest exporter of steel to the US is Canada at around 17% of the total, or 5.5 billion tonnes in 2017, followed by South Korea, Mexico and Brazil. China exported 1.9 billion tonnes, although this number may be much higher, considering that the country often flouts trade rules by sending steel and aluminum to the US through third countries.