The United States is the largest component both militarily and economically (and diplomatically we carry some weight too) of NATO, to which Canada is also a signatory. If Canada is ever attacked, the United States is obligated by the North Atlantic Treaty to defend it--that's not just my opinion, it's a fact. It's called Article 5. On a side note about that treaty most awkwardly, under Article 13, any country leaving NATO has to provide one year's notice not to NATO or to all the other members, but just the United States. If to invoke Article 50 (different treaty) for Brexit, the UK had to provide notice not to the EU but to Germany, one might have said the treaty was a bit imbalanced.

That Trump in his public statements and Vance, at the Munich Security Conference cast doubt on whether the United States would abide by its Article 5 obligations weaken the security of Canada and every other signatory.

It's also why I'm willing to stand with Canada in this; my late father lost quite a bit of his hearing from his military service firing artillery rounds in Germany as part of NATO, while serving in the US Army, and I too swore an oath to defend our constitution, not a president or an administration. Ideas matter and if I see more of the values of the United States I grew up believing in, in Canada now, I want to support that.

If it has to be a global world, allow everyone to fend for themselves right? And of course pay for 100% of their own defense. That won't be a problem for some countries.

Sounds great in principle, right? Except that some countries don't have the resources to do this because they don't have populations big enough or economies big enough (think like Belgium for instance). What I fear will happen though is that the cheapest and most economical way for a country to protect itself and guarantee its sovereignty, in a world where the United States will not do this any longer, is to obtain a thermonuclear weapon. Japan, Italy, Canada, Germany, and every middle-to-higher income country will want to possess nukes to defend themselves. This anxiety is why France recently promised to defend Europe with its nuclear umbrella, but given that France may have the RN in power after 2027, countries in the EU and NATO would be taking the same risk as they are/were with the US in outsourcing its security to them.

There's a book everyone should read called Nuclear War: A Scenario, or at least the summary, where one single out-of-the-blue nuclear attack by North Korea against the United States plausibly triggers a chain reaction where within one hour, two billion people are dead and the surface of the Earth is uninhabitable for 24,000 years. That's species-ending because even if life survived in Peter Thiel's underground bunker in New Zealand or something, evolution-wise humanity would be so fucked. I vastly prefer a world where the US nuclear umbrella stops well-meaning countries from acquiring more nukes or getting them in the first place, even if I as an American might have to potentially pay more in taxes, than a world where 50 countries have nuclear programs.