President Donald Trump signed an executive order Saturday imposing a 25% tariff on imports from Mexico and Canada and 10% on imported Chinese goods, according to multiple reports, reflecting his campaign promises to implement high tariffs, against the views of many economists who believe Trump’s plan would burden everyday consumers.

Trump’s move to impose new tariffs has bewildered the United States’ closest allies and trading partners, who have vowed to respond with levies of their own.

Uncertainty surrounds the trade war touched off by Trump’s tariffs. But some of the basics are taking shape.