It’s the Senator’s grandfather. He died four days after the Japanese surrender. John Sidney McCain II was also a Navy admiral, John Sidney McCain III is the Senator, John Sidney McCain IV is a graduate of the United States Naval Academy.
Some notes from wiki:
John Sidney “Slew” McCain Sr. (August 9, 1884 – September 6, 1945) was a U.S. Navy admiral.
He was the father of Admiral John S. McCain, Jr.; they became the first father–son pair ever to achieve four star admiral rank in the U.S. Navy. He was the grandfather of U.S. Senator from Arizona and 2008 Republican presidential nominee Captain John S. McCain III, and the great-grandfather of John S. McCain IV. All four generations graduated from the United States Naval Academy.
William’s son, the first John Sidney McCain, known as J. S. McCain (which may explain the apparent discrepancy in Senator John S. McCain being the III, rather than the IV), served as Sheriff and, later, President of the Board of Supervisors of Carroll County.
Interestingly, he is technically John Sidney McCain, II. His father was also John Sidney McCain. The family uses the designations incorrectly as noted above.
I also find this amazing: All four generations graduated from the United States Naval Academy. I looked up grandpa to see if his father had been a military man, but he was not having been a Mississippi plantation owner. His grandfather, William Alexander McCain died while serving as a private in Company I, 5th Mississippi Cavalry Regiment, Confederate States Army.