WWI was, indeed, a senseless pissing contest between privileged, inbred cousins. But the problem with the 500-mile border is that General Smedley Butler was born in 1881 and the world was a much different, and much, much bigger place then. Even after WWI no one could have imagined the horrors of WWII.
Today an 'enemy' can destroy ones entire country in a matter of minutes from the other side of the planet. Also, with international trade and banking a nations economy can be held hostage or destroyed by a nation on the other side of the planet. Like it or not, today were are all inextricably tied together. Look at how, today, the US & China have - as the Boss would say - "tied faith between our teeth" because of our intertwined economies and antagonistic military stances.
Today, what happens and what people do on the other side of the planet matters.