Mr. Waltz, who was a senior research scholar at Columbia, was a pre-eminent thinker who was known for his contrarian, debate-provoking ideas. Kenneth Neal Waltz was born on June 8, 1924, in Ann Arbor, Mich. He served in the Army during World War II and then earned a bachelor’s degree in economics from Oberlin College in Ohio. He studied political science as a graduate student at Columbia, and his dissertation was published in 1959 as a book, “Man, the State, and War: A Theoretical Analysis.” His “Theory of International Politics” (1979) advanced understanding of the concept of a “bipolar” world, with two dominant powers. Though he did not invent the concept, he showed how it worked. The book became a standard text. In 1995, Mr. Waltz and the Stanford University scholar Scott D. Sagan published “The Spread of Nuclear Weapons: A Debate,” which also became popular in international relations courses. They updated and expanded the discussion in subsequent editions.