Here is the thing. Ukraine has, or could easily within weeks have, nuclear capability (they were actually instrumental in developing Russia's nuclear capability, and they still have the infrastructure, knowledge, and skills) and that is actually to their detriment. It only promotes a 'first strike' mentality in the Russians.
The land border between Canada and the US is just too long for either side to defend.
But here is the thing. The only way the American army could have the resources to do it would be through the National Guard. American forces are spread too thin throughout the world. The National Guard is under the control of the individual state.
The biggest states would be well prepared to sign non-aggression treaties with Canada, meaning their National Guard could not be used. The individual States can not directly sign a treaty, but they CAN prevent their National Guard from participating. If Trump tried to take charge of them, it would for sure lead to a Constitutional Crisis. Since the Commander-in-Chief of the National Guard Units is the governor of each state, not the President, the troops are only answerable to the chain of command that ends at the state level. The individual states would have all the reason they need to secede. It would be civil war second inning in America. And some National Guard units have their own fighter jets and air bases.
That chaos is Canada's best bet for defense.
America never was, really, a 'democratic' country in the sense that so many were always denied the right to vote. It was 'democratic' only in the sense that those who were enfranchised to vote, could exercise that franchise given that they met certain conditions.
The original Constitution only enfranchised white male landowners to vote. If you met that threshold, you had the 'democratic right to vote'. Oh, wait, you also had to meet a list of other requirements, as well. For instance, you could be any religion you wanted, as long as it had God at the top. It was imperative to swear an oath to God. The original Constitution made that clear. No atheists or non-God-fearing need even try to vote. THAT was what they originally meant by 'freedom of religion' - freedom to worship God any way you wanted. It did NOT allow for 'freedom to NOT have a religion'.
You are right in the sense that the Constitution is being ignored. That is all part of Vance and the 2025 Manifesto. Trump will go one step too far, Vance will lead the impeachment procedures against Trump for Treason to the Constitution, Vance becomes President, and gets a totally free hand to implement his own 2025 Manifesto agenda. Incidentally, the splitting up of America leaving only a pure Republican union, ensuring the Democratic states never interfere again, is all part and parcel of the 2025 plan. Vance would rather be President of a 'pure' Republican nation than have to contend with the turmoil and chaos the Democrats would cause. That is why America is as close to civil war as it was before the start of Civil War 1.0 back almost two centuries ago. The fact is, the more the Constitution loses relevance, the more likely succession by states becomes. At some point, the Constitution becomes so irrelevant that individual states will see absolutely no need to follow it in any shape or form. They will fall back on State constitutions for their governance.
I predict, that soon the Republicans will use the turmoil over the existing Constitution, seeing as the existing Constitution is failing so badly, as a reason to completely rewrite the Constitution as they feel it should have been written And every person who points out how badly the current constitution is failing, just feeds into that argument, and America comes all that closer to ensuring a Vance victory.