If this happened as described the person who did this should be handed over for trial in Mexico. Which isn't going to happen. Maybe the Mexican government should send their army to the border to protect their citizens.
It might have been an accidental firing, it may have been some retard racist, it may have been twenty other things. Either way, two friendly neighbours that need each other's economies would not and should not start murmuring about war over relatively small incidents like these.
Yes, it hugely sucks for the victim and the US should financially pay for everything but that is what you have diplomats for.
It would be a casus belli, but it won't actually result in war. There's a difference. No matter what the situation, unless the US gives the shooter over to Mexico, it's a perfectly justified casus belli, just a useless one because there is no way that's happening and more diplomatic ways to handle it.
Eh, no, it's not. Any idea how many people are killed by stray bullets, each year? Those parties where people start shooting in the air... Those bullets have to come back down at some point, and they do, and unfortunately too often on people too.
It may have been on purpose, it may not. Claiming it's impossible is nonsense
And that war is one of the first examples always brought up of needless war in an era of rich people looking for glory as a distraction from boredom and a way to gain social rank.
Not really a great example for this considering the refrain after was "never again".