Canada and Mexico have been excluded from President Donald Trump’s tariffs on steel and aluminum imports.U.S. President Donald Trump signed proclamations hammering global steel and aluminum imports with tariffs of 25 and 10 per cent.The tariffs take effect in 15 days.Speaking in a briefing, a senior administration official says there’s no end date set on the exclusions.The official, speaking on condition of anonymity in order to discuss matters in advance of them being made public, would not speak to the issue of whether the threat of tariffs would be used to bully Canada and Mexico at the NAFTA bargaining table, saying only that NAFTA is important to economic and national security.The tariffs are being cast as a national security matter.A provision in U.S. law allows the president to set emergency tariffs if it’s a security issue. But the White House had been undermining its legal case in recent days. With the move sure to prompt international lawsuits and counter-measures, it’s been describing the tariffs as a NAFTA bargaining ploy.The White House is now avoiding that kind of talk.It’s also denying reports that it picked the tariff levels arbitrarily. The initial recommendation to Trump was for tariffs of 24 per cent on steel and 7 per cent on aluminum, but news reports have described the president seeking round numbers.The White House official who delivered today’s briefing says it was only upon careful consideration that Trump settled on those larger, rounder numbers of 25 and 10 per cent.He did not explain how that version squares with the fact that the round numbers remain in effect, despite the entire formula being upended by the fact that major suppliers have now been excused from the tariffs.Canada is the No. 1 seller of both steel and aluminum to the U.S.The fact that Canada could be on the initial hit list had become a political sore spot for the administration, as U.S. critics of the move ridiculed it by zeroing on the idea of national-security tariffs against a peaceful next-door neighbour and defence ally.