The first mechanism is the ability of certain countries, with which the US have security relationships, to be exempted completely as long as they can assure the US Administration that there are "alternative ways to address the threatened impairment of the national security caused by imports from that country".
Other countries can apply for exemptions, according to the Trump administration, although details of when they would be granted were thin.
But the drama isn't over. It's hard to see how the conclusion of a new NAFTA deal would alter the security situation, suggesting the president's linkage between an exception for Canada and Mexico and the conclusion of a renegotiated NAFTA agreement has little to do with national security and more to do with leverage and protectionism.
Trump's top economic adviser Gary Cohn has resigned in apparent protest over the controversial tariffs. It's gonna give us ... the opportunity to negotiate a great (NAFTA) deal for this country. "And if we get that, all's good with Canada and Mexico".
Trump announced on Thursday that tariffs on steel and aluminium imports will start in 15 days with initial exemptions for Canada and Mexico and the possibility of alternatives for other countries. It is the biggest break in Republican unity during the Trump administration so far. Republican leaders in Congress, meanwhile, know full well that their troops do not want a battle, and so they are not doing anything to rally them for one.
The Canadian government isn't celebrating yet.
"Rio Tinto has recently been growing its market for Kitimat aluminium in the USA, and given that, ironically, our countries are now attempting to renegotiate a new fair trade agreement under NAFTA, this move by the U.S.is troubling to our members".
"I'm concerned that after everything we've done to help our constituents through tax reform, the tariffs the president recently announced are going to undermine some of these recent gains", said Rep. Kevin Yoder, Kansas Republican.
Behind the scenes, a full-court, 11th-hour diplomatic press was underway Wednesday. "Hitting China and Canada with the same tariff doesn't penalize China relative to anyone else", says Dan Ikenson of the Cato Institute. Countries have filed complaints about national security tariffs, but either the tariffs were dropped or the cases never proceeded.
Canadian Foreign Minister Chrystia Freeland, meanwhile, called the announcement a "step forward" and said Canadian officials had exerted tremendous efforts to get the exemption.
The move will indeed create U.S.jobs: an aluminum producer in Kentucky has already announced plans to recall 300 workers and a steel plant in IL will bring back about 500 employees.
Trudeau, meanwhile, spoke with the president this week.
"A source familiar with the last-minute scramble likened it to a high-stakes, reality-show contest, with a drama-courting US president at the centre of the production: "(It's a) last-episode-of-'The-Apprentice' kind of thing".
However, Levy makes the very appropriate point that the new steel and aluminum tariffs are being pursued under Section 232 of the Trade Expansion Act of 1962, in which the provision deals with instances where imports threaten the national security of the U.S. It would seem that the Commerce Department has come to the conclusion that steel and aluminum imports do pose a threat to the viability of the U.S.'s steel and aluminum industries.
The biggest American business lobby is among those fighting the plan.
"Exempting Canada and Mexico is a good first step, and I urge the White House to go further to narrow these tariffs so they hit the intended targets - and not U.S. workers, businesses, and families", House Ways and Committee Chairman Kevin Brady, who is a Republican, said in a statement.