From The Lord of the Rings
Denethor I lived in the Third Age, 2375 until his death in 2477. He was the tenth steward of Gondor and is known as the last steward who saw the end of the Watchful Peace. 25 Years after Denethor I took the title of Steward of Gondor, Sauron returned to Dol Guldor in Mirkwood. The peace ended for Gondor when orcs pillaged Ithilien and took over Osgiliath. Denethor's son, Boromir, recaptures both cities but Osgiliath is completely destroyed. The city of Gondor does not see peace until after the War of the Rings ended.
To avoid any confusion, the Denethor and Boromir mentioned in The Lord of the Rings were actually the second people to bear those names. The Denethor in the book was the twenty-sixth Steward of Gondor.