I must disagree, it took considerable strength of will to throw off the Ring's spell. IMO he was vulnerable because of his desperation and/or ambition was not tempered, (as with Aragorn and Faramir) by an understanding of the Ring's nature.
And yet Denethor, who surely did have the lore to understand the peril of Ring, was falsely confident of his own power to resist. Boromir is clearly less blameworthy in that respect than his father.
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