Robert D. Stuart Jr. at home in Lake Forest, Ill., by a portrait of himself when he was 4. Mr. Stuart was ambassador to Norway from 1984 to 1989. (Jim Robinson/The Chicago Tribune)

Robert D. Stuart Jr., the politically active heir to the Quaker Oats Co. who led the company for 15 years and, as a student at Yale Law School in 1940, ignited the America First movement against U.S. intervention in what became World War II, died May 8 at 98.

He died while traveling from France to the United States, according to Truman Anderson, who directs the Stuart family’s Lake Forest, Ill.-based charitable foundation. Crain’s Chicago Business, citing his son, Sandy Stuart, said the cause was a heart attack.

Quaker Oats, maker of brands including Gatorade, Rice-A- Roni, Cap’n Crunch and Aunt Jemima, was bought by PepsiCo Inc. in 2001, the 100th anniversary of its founding in Chicago. Its roots go back to the 19th century, when several businesses merged to become American Cereal Co.

One of those businesses, based in Cedar Rapids, Iowa, was run by Mr. Stuart’s great-grandfather, John, and grandfather, Robert. His father, R. Douglas Stuart, served as Quaker Oats president.

As chief executive from 1966 to 1981, Mr. Stuart oversaw the introduction of instant oatmeal and Quaker Chewy Granola Bars. He also continued the Chicago-based company’s growth beyond oatmeal.

The 1969 purchase of Fisher-Price Toy Co. of East Aurora, N.Y., was Quaker Oats’s first non-food acquisition since 1942, when it had expanded into dog food by buying Chappel Brothers Inc., maker of Ken-L Ration.

Quaker Oats turned Fisher-Price into an industry leader through national television advertising. Explaining the acquisition of the toymaker, Mr. Stuart told the New York Times, “The back of the cereal box on the breakfast table just seemed to be a logical fit between the cereal and toy businesses.”

Fisher-Price sales rose to $300 million in 1980 from $25 million in 1966, according to the Times. By 1990, though, the toy unit was being blamed for earnings declines, and Quaker Oats spun it off in 1991. It has been a unit of Mattel Inc. since 1993.

Mr. Stuart was U.S. ambassador to Norway from 1984 to 1989, appointed by former president Ronald Reagan.

Politically active throughout his life, Stuart was an Illinois Republican Party committeeman from 1964 to 1972 and served on the national Defense Base Closure and Realignment Commission in 1991 and 1993.

His first leap into politics made history.

At 24, he became founding national director of the America First Committee, the antiwar movement begun in 1940 at Yale University in New Haven, Conn., where he was attending law school.

Other Yale students involved in its formation included Gerald R. Ford, the future president; R. Sargent Shriver Jr., who would become founding director of the Peace Corps; and Potter Stewart, a future Supreme Court justice.

In its first story on the movement in November 1940, the Times quoted Mr. Stuart as saying that the push for U.S. intervention against German aggression “seems to come from those who want to go and save England. We are taking a stand for unity in defense and peace.”

Though the America First campaign went down in history associated with the anti-Semitism of aviator Charles Lindbergh — “The three most important groups who have been pressing this country toward war are the British, the Jewish and the Roosevelt administration,” he said in a September 1941 speech — Mr. Stuart said its founding core consisted of Democrats and Republicans, Jews and non-Jews who shared reasoned viewpoints against war.

“Most of us in our generation who were in any way thoughtful about history and international affairs learned that the U.S. didn’t accomplish very much in committing troops to the First World War, which was a terrible slaughter of the talent of the Western world — an internecine conflagration,” Mr. Stuart said in a 2000 interview with Bill Kaufmann, editor of “A Story of America First,” a 2002 book by Ruth Sarles.

The Japanese attack on Pearl Harbor Dec. 7, 1941, followed days later by Germany’s declaration of war against the U.S., spelled a sudden end to America First. Mr. Stuart, who had taken a leave from Yale to run the committee, enlisted in the U.S. Army and fought in Europe.

He told Kaufmann the America First founders never bothered to have a postwar reunion. “We may be a little sensitive to the fact that the world still thinks we’re the bad guys,” he said.

Robert Douglas Stuart Jr. was born April 26, 1916, in Hubbard Woods, Ill. His father was U.S. ambassador to Canada under President Dwight D. Eisenhower and Quaker Oats president.

The younger Stuart graduated from Princeton University in 1937 and Yale Law School in 1946. He joined the family business in 1947 as a sales trainee in Los Angeles.

He had four children with his wife, the former Barbara Edwards, who died in 1993.

— Bloomberg News