President Donald Trump speaks during a meeting with leaders of the steel industry at the White House March 1, 2018 in Washington, DC. Getty Images

To implement new taxes on imports of steel and aluminum, President Donald Trump is relying on a law last applied in the early 1990s. The law – specifically, Section 232 of the Trade Expansion Act of 1962 – allows the president to bypass Congress and impose tariffs by executive order. In this case, Trump made his determination on Thursday, announcing with little prior indication at a White House meeting that his administration would impose tariffs of 25 percent on steel and 10 percent on aluminum. The legal justification for the tariffs came in the form of a report from Commerce Secretary Wilbur Ross, which outlined the national security threats from the imports in question and provided a framework for retaliation. Ross' report originally suggested a 24 percent tariff on steel; Trump nudged it up to 25 percent. "Your guess is as good as mine" for why Trump increased the number, said Edward Alden, a senior fellow at the Council on Foreign Relations, a nonpartisan think tank. "Twenty-five sounds better, probably." The Commerce Department did not immediately respond to CNBC's request for comment.

Executive tariffs

What can Congress do?