In more facist periods the bourgeois ideology gets more and more irrational traits to try to convince segments of the petite bouegeois and the workers to support the bourgeois state despited the increased exploutation of the working class, this ultimately reflects in more irational leadership as well. In this case, I think it is entirely possible the US leadership too eagerly bought into their own propaganda about their military strength and the supposed weakness of their enemies. It is not because of Trump or anything like that, he is just part of a less rational bourgeois state apparatus due to material reasons.

That being said, I think they may have had some indications that could have implied that the war would go better for them, which they too eagerly accepted.