President Donald Trump signed a proclamation authorizing the tariffs on imported steel and aluminum, while excluding Canada and Mexico and leaving the door open to sparing other countries on the basis of national security. Accordingly the U.S. will levy a 25 percent duty on steel and 10 percent on aluminum, and the tariffs will take effect in 15 days.

Trump’s authority to establish the tariffs stems from a Commerce Department investigation that found that imports of the metals pose a risk to national security. The probes were authorized under the seldom-used Section 232 of the 1962 Trade Expansion Act, which gives the president broad powers to impose trade restrictions on domestic security grounds.