Early story: Trump administration files WTO complaint against Canada's retaliatory tariffs, arguing its own steel and aluminum tariffs are permissible but Canada's pushback is not: thestar.com/news/world/201…
In response to the U.S.'s WTO dispute against Canada's tariffs, spokesperson for Minister Chrystia Freeland repeats what Freeland has said: the U.S. tariffs are illegal, U.S. "national security" grounds are absurd, Canada's tariffs are reciprocal, they'll stand up for Canadians. 
Here's a brief explanation of the Trump administration argument that Canada's retaliatory tariffs are illegal. 1) Countries are allowed to impose "safeguard" tariffs to temporarily protect certain industries under certain conditions. Other countries can retaliate.
2) Canada and other countries say the U.S. steel and aluminum tariffs are safeguards (improper safeguards), so they're allowed to retaliate. 3) The Trump administration says they are not safeguards, they're national security tariffs against which no retaliation is allowed.
In summary, the Trump admin is not only using a "national security" provision of U.S. trade law to impose these tariffs - it's using a separate national security provision of international trade rules to argue that nobody can retaliate against the tariffs.
