There is a certain irony to the fence building fervour of our neighbours to the south.
There was an official Canadian government plan to invade the U.S. Called the Defence Scheme No. 1, it was to be implemented when yet another invasion by American forces seemed imminent. There was no illusion on the part of the drafters of this plan that it
was anything more than a forlorn hope to try to keep enough property together to wave a flag over until Great Britain and our Commonwealth allies would show up to save the day.
Actually, a fence sounds like a damn good idea. The first act of bellicosity by our southern sibling after unpacking the Continental Army was to point it northward. It wasn’t an action against ‘Canada’, mind you. Canada wasn’t even a name on the map yet. They were coming to the British Province of Quebec with an offer to liberate the population from the maudit overlords.
Oddly enough, the only residents of Quebec interested in the offer were former residents of the 13 colonies who had migrated across the river.
The Yanks did manage to occupy Montreal but, like invaders before and since, neglected to take a quick look at the calendar before marching on to Quebec City. In a snowstorm, on the 31st of December, 1775, the combined forces of General Richard Montgomery and General Benedict Arnold (yes, that Benedict Arnold) found themselves on the wrong side of the famous (and still standing) walls.
Montgomery was killed in the initial engagement and Arnold was wounded. This was followed by a seige of 5 months in which the shivering, starving American forces were 
subjected to unrelenting winter and derisive gestures from the Caribou drinking Carnival party-goers dancing with Bonhomme on the ramparts.
Meanwhile, in Montreal, the American occupying forces managed to alienate both the former yankee colonists who had helped them get in the door and the French speaking Canadiennes they were there to save.
“We are here to free you from the yoke of British rule that respects your religion and language rights so you may experience the freedom of being assimilated into our manifest uniformity of fundamentalist protestantism and superior english language.” Oddly enough, the Quebecois were somewhat cool to the idea.
Come the warmer month of May, the British launched a counter-offensive sailing down
the Saint Lawrence from their stronghold of Halifax and lifted the siege fairly quickly. If ever an army could be said to engage in a grateful retreat, it was those poor sods still tottering on frostbitten feet when the White Ensign hove into view.
Like “yes” to the question “Are you with the band?”, “Let’s invade Canada” became the right answer for U.S. leaders needing to demonstrate decisiveness and resolve by spanking some British red-coated backside.
From the War of 1812, where thousands died for no significant gain to one side or the other, to the Pig War of 1859 in which the only victim was the pig (well, it did get a 
place in history but that’s fairly cold consolation, I’m pretty sure) to the occasional well intentioned thuggery by Fenians raiders, the border between the U.S. and Canada has been a welcome mat in the north bound lane.
The Mexicans may have their own reasons for wanting to build a wall but, in their case, it’s just a bit late to start digging post holes around California, New Mexico and most of Texas…
What’s that saying about strong fences and good neighbours? Maybe I’m thinking of “better late than never”?.