The US does not prevent wars. It is a cause of most of the wars in modern times. It has done something like 100 military invasions. Toppled countless democratically elected leaders and undermined democracies.
US hegemony is to set be the US, not to serve the world.
Yes, that's the argument, in a hegemony (usa or otherwise), wars basically only happen when the hegemon wants them to, and the hegemon has so much soft power its rare that they want them to, and when it does happen its small scale.
The end result is ordinary people get caught up in less wars because politically war is much more rarely the right answer (either you're too politically weak to start one, or if you are the hegemon you just give people the evil eye and they usually shutup)
That doesn't make it a utopia, just a world with less war dead.
Vietnam was the US standing to a foreign power trying to take over. It is not a war on the nation, but a war on an aggressive faction. Same for Korea really.
The Vietnamese fought for freedom from colonialism. The got that from the French, almost, until the US intervened. The only aggressive factions were the French and the US.
Korea was different, and also backed by the UN. But even there things aren't as clear cut as you imply. North Korea had yet to turn into the Kim-led hard core Stalinist regime.
You yourself just wrote a the answer: it prevents wars.