Denethor's madness.

[Eomer of the Rohirrim]

I will. It was illogical.

Anyway, that's for the Faramir thread. As for Denethor, he was basically just a jerk all the way throughout. A real shame I thought.

In the film, Denethor seemed to be in denial about Gondor's ridiculously low chances in the War. Then he sees Mordor's army, his eyes open up (literally) and he goes insane. I think that is as central an explanation as any, within the film's logic.