Atanatar II was the sixteenth King of Gondor.
During his reign, the crown of Gondor was changed from the Helm used by Isildur in the Last Alliance, to the crown Aragorn Elessar was crowned with in "The Return of the King".
Atanatar ruled Gondor at the height of its power. Gondor was so great that "men said precious stones are pebbles in Gondor for children to play with."
Unfortunately, Atanatar was lazy, and loved ease. He did not maintain the power he had received. As did his children.
"The waning of Gondor had already begun before he died, and was doubtless observed by the enemies."
The next war wasn't caused by Mordor, but by Gondor itself: The Civil War that happened around three-hundred years later caused great destruction that was never fully repaired.
Atanatar II, or Alcarin (the Glorious) as he was called among the men of Gondor, was succeeded by his son, Narmacil I, who was the second childless king.
King Narmacil was succeeded by his younger brother, Calmacil.