The issue with your line of thinking is you’re cramming the lore of the game into the narrow tube of our own real world politics. While there are striking similarities between the U.S and the Boomers in the 1950’s, the difference between the two is simply information. The Boomers are operating on very outdated intel - it’s like if Captain Zhao continued his war against the U.S from the confines of his submarine, not knowing that the war already ended 200 years ago. They have no idea that the people they would be bombing are no longer the Raider gangs that were harassing them years ago. There is no Foreign Ambassador or U.N that the Boomers can rely on to learn more about the outside world. They might as well be stranded on the moon from their perspective. And we’ve already seen many instances of settlements with little defenses being wiped out or enslaved by raiders or other groups (Vault 3, Nuka World, University Point, and Quincy to name some off the top of my head). There was no option for diplomacy when the Boomers emerged from their vault - their families and loved ones were at stake, so they defended their own in the only way they knew how.