Canada

Switzerland? Apart from a bit of aerial bombing during WWII it's been a pretty peaceful border since that time Napoleon invaded.

The US and Canada haven’t fought since 1812. Which European border of a major super power has a longer history of being close allies?

What’s the definition of “being a close ally”? You’re using the date of Canada and USA last conflict, but for Spain and France you’re using political alignment.

I think Portugal and Spain also make a good candidate if we’re looking back only until the early 1800’s. The border itself had a few changes but they were peaceful IIRC, the last conflict was 1801?

On a separate note, the quote says has been the most peaceful and beneficial, so it’s not so much as a matter of peaceful for the longest time. Even if EU borders weren’t peaceful way back, quite a few of them are so peaceful nowadays that they barely register as existing. In terms of most beneficial, I’m not sure how to analyze that.

Quite a surprising one here: I think the Norwegian/russian border can actually match that. I believe Norway is the only country neighbouring russia that has never been invaded by them (sans WWII, where they invaded Nazi-occupied Norway and willingly left after the Axis was defeated).

I also think the Norwegian/Danish border has been conflict-free for some hundred years (to be fair, we were in a union for ≈450 years ending in 1814). We've had some skirmishes with the Swedes throughout the years, but I believe the last one was in 1814.