At what point did the prior abuse the Garleans suffered get used as a means of justifying the current war? This has not, as far as I know, been an assertion anyone in this thread has tried to make. I am observing the reason why I believe the Garleans might, in their own minds, feel justified in using Black Rose when push finally comes to shove. Not entirely sure how people are getting that I'm actually justifying it. I nor anyone else has made the claim that the Garleans are being rational or that what they may do is anything resembling good idea. I am simply pointing out that desperate people, particularly those that have been indoctrinated into a certain line of thinking, are prone to doing incredibly stupid things.
The horrors inflicted upon their race for God knows how long before they formed their Empire, not to mention Emet-Selch's eventual meddling during and after that Empire's foundation, in all probability has a lot to do with their warped philosophy and the ease with which I believe they could become desperate enough to do a dumb. Abject desperation, as mentioned before, is not something that leads to rational decisions being made. Surrender suddenly stops looking like a viable solution when you're convinced that your entire race might be exterminated as a result of it. They're not desperate yet, but to think the Garlean propaganda machine would be above trying to convince the population of their impending annihilation if/when the war goes completely south for them is most likely in error.
That aside, what does the American incursion into Vietnam have to do with Garleans being nearly wiped out by the other spoken races in the past? I'm not seeing the correlation here. The Garleans didn't conduct any invasions or take to acts of provocation back then. They did, however, begin making extensive use of subterfuge and diplomacy after managing to assemble a few of their remaining tribes into a cohesive society of sorts. Happened long after they were driven into the harsh lands they now inhabit by their persecutors, though.
I also feel that your comparison of Eorzea and the Empire to American revolutionaries and the British Empire to be a bit disingenuous. Both the Garleans and Eorzeans have ample means to traverse long distances in an exceedingly short period of time. It would not be difficult at all for one side to renew hostilities with the other. Garleans have their technologically advanced airships and weaponry, the Eorzeans have their own airships and a whole host of people that are capable of using magic to varying degrees. The Americans and British, however, could not easily take shots at each other during the period of the revolutionary war. It took them considerably more time and preparation to move troops and assets over large distances - especially something like an ocean - than it would the Garleans or Eorzeans.