He was wrong. III 2460, the twenty-fifth of Denethor's rule, would be last year to see Gondor fully at peace for nearly six centuries. In that year, Sauron gave up his exile and returned to his fortress of Dol Guldur in Mirkwood. Just a few years later, it was seen that the Nazgûl had not been idle in the years of the Peace - they unleashed a force of hideous soldier-orcs on Gondor, of the kind that was then unknown, but would become known as the Uruk-hai. They ravaged Ithilien and took Osgiliath, but Denethor's heir Boromir fought them back and recovered Gondor's holdings east of the River, though he was dreadfully wounded. These were the first skirmishes in a conflict that would not truly end until the time of the War of the Ring.
Denethor I ruled as Steward for forty-two years, and was succeeded by his son Boromir.
The date of Denethor's birth appears only in The History of Middle-earth volume XII, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.