Denethor was wise and restrained, but Sauron held the master Palantir and was able to slowly corrupt him (as he did Saruman).
Not to be picky, Sauron had the Ithil-stone, The master stone was lost in the fall of Osgiliath.
His first action as a proclaimed heir was to go to the Stone of Erech and claim the fealty that the dead owed Elendil.
Isildur instead of Elendil
I thought that the Ithil stone was brought to Minas Tirith after the fall of Minas Ithil and that Sauron acquired the master stone with the fall of Osgiliath. Which stone did Denethor have if not the Ithil stone? My library isn't handy right now.
I thought that Elendil called them to fight by his side at the last alliance of Men and Elves. When did Isildur in his short reign call on them?