I'm not sure trying to improvise a way through a novel kind of conflict counts as stupid so much as untrained. Maybe there was someone in WWI saying "This is obviously wrong and won't work - we should do this other thing instead."
Maybe there wasn't. At this distance it's hard to tell.
We tend to call someone stupid when the quality of the predictions is worse than some population median, and the self-harm seems both unsurprising and trivially avoidable.
Stupidity is only loosely related to IQ. Low IQ people don't have the horsepower to make good models, so they're often blindsided by challenges. But high IQ people can have bad beliefs and poor models. Without self-correction and a reality-testing loop they're just as likely to believe wrong and dangerous things.
One of the most toxic things that happens in politics is re-framing a problem away from rational competence towards emotive and narrative non-logic. If you can succeed in this, you can make people stupid. Emotion and narrative, especially ego-gratifying and tribal narratives, are reliably unrealistic models that lead to poor predictions.