This is pretty wild. Trump is saying that his steel and aluminum tariffs, which are officially being imposed on "national security" grounds, are actually a response to Canada's milk protectionism. 
The G7 countries found a way to come up with a joint statement that papered over their vast differences with Trump. Trump is now saying he's rejecting the statement because he's angry at Trudeau's (quite restrained) news conference.
As with the Russia investigation, the travel ban and lots of other things, Trump is saying things out loud about his tariffs that undermine his administration's superficial official rationale and that undercut his legal case.
Also, this is the first time Trump has linked his possible "national security" auto tariffs to his unhappiness with Trudeau and Canada. Such tariffs would blow up a central part of NAFTA, be a huge deal for a North American industry way bigger than steel and aluminum combined.
Trump is right on this point: Canada has a 270% milk tariff. It's part of Canada's extremely strict protectionist dairy system called "supply management." Polls suggest Canadians have mixed opinions on it, but the dairy lobby is powerful.
The Canadian dairy industry’s response to US complaints is that the US dairy industry is heavily subsidized by governments. Trump omits any mention of these US practices. But he’s accurate about Canada’s hefty tariff. realagriculture.com/2018/02/u-s-da…
Anyway: we could argue about milk all day if we want to have a non-fun day, but the bigger point: all signs are that Trump has seized on this milk issue as a pretext for his desire to blast Trudeau in the wake of Trudeau’s criticism of his steel and aluminum tariffs.
...And now he’s gone so far with this pretext that he’s claiming Canada’s milk tariffs are not only a problem but *the reason he imposed steel and aluminum tariffs* on Canada, which makes little sense and further renders his “national security” rationale a sham. OK bye guys
President of Canada's auto parts industry association:
This was the subdued, sorry-but tone of the Trudeau news conference that so irked Trump:
