And we haven't had any serious threats from Canada since 1812. I think the most reasonable estimate is 100-200 years
I could also take the example of world wars, in France ww1 was deemed "la der des ders", which meant "the very last war" or "the war to end all wars", well 20 years later we were at it again
Or simply look at China, you don't even have to go back 100 years in the past to see drastic changes.
I'm not claiming nobody will invade France or Taiwan in the next 100 years, I'm claiming that the US is special. We haven't been invaded since 1812 and haven't really been attacked since 1941. It's reasonable to predict we won't be invaded or go to war with our neighbors for 100 years since it hasn't happened for 213 years
My own death has not yet been a problem for me, but I can safely assume it will be.
Mexico was dealing with its own internal issues (revolution) which made it difficult for them
A slight turn of events and the us would have a huge southern front I deal with, and a base of support for disaffected native Americans and African Americans.
Russia also dropped out of Wwi due to its intern revolution
It is easy to look back and see manifest destiny as a given. There were a lot of contingencies
"War" as a concept does not have to include two militaries fighting each other.
When you stretch the definition of war to absurdity, so can I.