“For thy hope is but ignorance. Go then and labour in healing! Go forth and Fight! Vanity. For a little space you may triumph on the field, for a day. But against the Power that now arises there is no victory.” -The Pyre of Denethor, Return of the King

Denethor knew only despair.  He had seen the future and he had heard from Faramir himself on how he sent two halfings to their doom. Did he not know how risky it could be? He ran his strategies in his head more than once and he knew that time was running out.

Gondor was doomed.

Now his son burned with fever that would not abate. He was dying and his turn was soon to come. It was only a matter of time that they all perished.

The Palantir showed the reality. Gondor was awash in rubble. The Enemy had won.