Sauron corrupted Denethor

"It may noted that the effects were different. Saruman fell under the domination of Sauron and desired his victory, or no longer opposed it. Denethor remained steadfast in his rejection Sauron, but was made to believe that his victory was inevitable, and so fell into despair. The reasons for this difference were doubt that in the first place Denethor was a man of great strength of will, and maintained the integrity of his personality until the final blow of the (apparently) mortal wound of his only surviving son. He was proud, but this was by no means merely personal: he loved Gondor and its people, and deemed himself appointed by destiny to lead them in this desperate time. And in the second place the Anor-stone was his by right, and nothing but expediency was against his use of it in his grave anxieties. He must have guessed that the Ithil-stone was in evil hands, and risked contact with it, trusting his strength. His trust was not entirely unjustified. Sauron failed to dominate him and could only influence him by deceits." - Unfinished Tales