Which horse won the 1978 Triple Crown in the United States?

Affirmed won the United States Triple Crown in 1978.

Affirmed captured the Kentucky Derby, Preakness Stakes and Belmont Stakes, becoming the eleventh American Triple Crown winner. His campaign is especially remembered for his rivalry with Alydar, who finished second in all three races.

The Belmont Stakes was the closest of the three contests. Affirmed defeated Alydar by a nose after the two horses raced side by side through much of the stretch. Their rivalry gave the 1978 series one of the most famous finishes in American racing history.

Affirmed was bred in Florida and owned by Harbor View Farm. He was trained by Laz Barrera and ridden by Steve Cauthen, who was only 18 years old at the time. Affirmed remained the last American Triple Crown winner until American Pharoah achieved it in 2015.