[z3phyr]

Thats true, but in the books, Denethor is competent and a seasoned strategist and has a battle of strategy with the witch king, one upping them in most instances, answering with brilliant maneuvers to the brilliant maneuvers of the witch king.

[conartist6]

Perhaps that's why Sauron's trick is to use the Palantir to show him some things while hiding others, so as to convince him that his every move would be futile.

I'm not sure even this is what destroys Denethor's mind though so much as it is the thought of the ring. He sees it as his by right of need. He sacrifices both his sons in his madness to have it, for the madness of power. His view of the world is so bleak that saving it in a way that destroys it seems "right" to him

[nkrisc]

That’s a good point, in the movie they only show the already “broken” Denethor.

[Ichthypresbyter]

In the extended edition of Two Towers they show him in a flashback of Gondor retaking Osgiliath.

It's not a particularly flattering portrayal- the military success is shown as belonging to Boromir more than Denethor- but at least it shows him sane.