In what sense is milk a vital need, though? Millions of people live without it.

So does the US. The vast majority of dairy farmers receive subsidies or they would not be "profitable", averaging $280 million per year over the past 20 years (some year it's $10M, while another its $1B). Everyday we dump over 250,000 gallons of milk. There is a reason why milk is cheaper in the US than Canada.

I don’t know enough about them to comment but situations like that is why trade agreements have tribunals and things go through the WTO etc etc. It’s a very complex balance with complex systems.

Canada also imports more than four times the dairy than it exports: http://www.cdc-ccl.gc.ca/CDC/index-eng.php?id=3803 (poutine 2-3 times per day adds up I guess).

Trump’s only direct authority for imposing steel tariffs is a narrow national security clause which is absurd and insulting to use against Canada and demonstrates that America can’t be trusted.

Even if a future president tries to rebuild international relationships I would be hesitant to waste years of effort working out any international agreement with America when the next TV populist president can easily tear it up. Unfortunately it takes a lot more effort to build things than tear them down.