The naivety there isn’t so much that soldiers would be incapable of fighting the US citizenry in a large scale war, but more that the framing of the question is false to begin with. It’s way easier for soldiers to commit small scale acts of terror than large scale genocides, and it’s always easier to commit acts of terror on minorities or the “other” rather than on the gen pop. If we were to see any domestic american guerilla warfare (I find this kind of unlikely compared to the rising amount of lone wolf, stochastic incidents), then it’s likely that even the regular population would get fed a ton of bullshit about the opposition being subhuman, or something to that effect. Larger scale versions of how, every time a black guy gets shot by the police, everyone trots out every encounter he’s ever had with the police within like 12 hours of the incident. Character assassination, but at a group level, instead of on the individual level.