Although the Trump administration has temporarily exempted Canada and Mexico from the upcoming steel and aluminum tariffs, that exemption could be ended at any time. If he ends the exemption and applies the 25% steel and 10% aluminum tariffs to Canada and/or Mexico, and assuming that he hasn't withdrawn the US from NAFTA (or the withdrawal hasn't taken effect) yet, would that violate either:
The NAFTA trade agreement between the US, Canada, and Mexico? and/or
The NAFTA Implementation Act passed by the US Congress?