When NAFTA was established, theoretically there was free trade throughout North America. Most products made in Canada, Mexico and America were made in those respective countries. Over the past 25 years things have changed dramatically. Canada and Mexico decided to stop manufacturing products made entirely in their countries, and start substituting products made in other countries, primarily China and the western Pacific Rim. In many cases now, products are only assembled in Mexico and Canada.
Lets use China as an example. America has not been able to get free trade with China because China sends their manufactured products to Canada and Mexico, assemble it, and get free trade into America through the NAFTA Treaty. China then puts a tariff on American made goods. Over the years this has led to a growing imbalance of trade. In 2017, America imported $507 billion from China and only exported $130 billion to China. This deficit is virtually all due to NAFTA.
Trump has tried to work things out with Canada and Mexico for over a year. Neither Canada or Mexico wanted to make things work. That led to the situation that exists today. The only way Trump can get around the House and the Senate is to declare steel and aluminum as vital to national defense. Trump isn't looking to take advantage of either Canada or Mexico. Just have free trade between each country individually. Canada's problem is that they don't make many products anymore. They buy things from other countries and assemble them, thus Canada is going to have a problem adjusting.
Regarding the EU.
@Red Leg covered this pretty well. America allowed Europe and Japan to have an advantageous trade balance after the war to help them rebuild. There is no need now to allow this imbalance to continue. Currently America imports $434 billion from the EU and export $283 billion. This is due to the EU having higher overall tariffs on products coming from America than America has on products coming from the EU. Trump wants free trade. Zero tariffs going each way. There is nothing sinister about this. Trump is not trying to get an advantage. Tusk, Junker and the other bureaucrats in the EU acted like no changes would be accepted, at least until Junker went to Washington this week with a change of heart.
All Trump is doing is keeping America from being taken advantage of. Since WWII by Europe and Japan, or since the mid 90's, by Canada and Mexico, due to NAFTA.
Canada has taken advantage of America since they started using foreign made products to export into America, changing NAFTA from it's original intent. This is all easily understandable with an hour or two of research. It is unfortunate that Freeland and Trudeau are inept. They are just making things much more difficult for the average Canadian. Hopefully Freeland and Trudeau will figure things out before things get to bad for our friends in the great white north.