No, they're borders that would be extraordinarily hard to defend, but conveniently it's not been relevant for about 200 years. Defense is about more than having enough machine guns on the border.

In the case of the southern neighbor and their neighbors, one might wonder if it's a complete coincidence that the collective Latin American shit hasn't been together enough to be a credible threat since that time they actually were and were eventually repulsed.

Diplomatic and, let us say, "special" methods of making neighbors safe to be nearby is one of the key advantages that made American soil defensible in a way that, say, early 1900s French soil wasn't.

The only credible way that border has been for the American century, or will be for the foreseeable future, a military danger would be if another superpower cut a deal to stage there, and that would be both incredibly obvious and would represent the world's longest and most vulnerable supply lines in the history of armed conflict.

The closest anyone ever got was missiles in Cuba, and it was so utterly disconcerting that there even was a threat in the same hemisphere that the world nearly ended.

You could also say that this only applies to conventional military threats, and there are other problems now. That is perhaps why things are beginning to feel different and sitting between two oceans and two pacified buffer zones isn't proving enough to engender national feeling, rightly or wrongly, of being large and in charge.

The US War Plan Red from the 1920s covers a war between the US and the British Empire and includes quite a lot about Canada!

https://en.wikipedia.org/wiki/War_Plan_Red

> a somewhat longer northern border that we've never actually had to think about defending.

Well, not since they (as lackeys of the Brits) burned down Buffalo.