Close to midnight on May 12, at the age of almost 89, Kenneth Waltz passed away. He was lucid until a just a few hours before the end -- mentally sharp and clear-eyed as ever, fully engaged intellectually in international politics (he always insisted on calling the field that, as opposed to “international relations”), worried about what he saw as America’s overextension, and insisting, as always, on the need to be wary about the use of U.S. military power.

Ken was the leading realist theorist of his generation, and arguably the leading international politics theorist of the last half century. His first publication, Man, the State, and War, came out in 1959 and remains in print today; the last thing he wrote, “Why Iran Should Get the Bomb,” appeared in Foreign Affairs in 2012. His teaching career spanned over fifty years and many institutions -- first Columbia, then Swarthmore and Brandeis, finally Berkeley, and then back to Columbia as an emeritus. In his scholarship, he asked the tough questions about the difficult and important issues. He was motivated by both theoretical and policy questions, the latter often setting the agenda for the former. He was intellectually courageous, usually staking out iconoclastic positions -- some of which were initially derided, many of which ended up becoming mainstream thinking. No matter what one’s theoretical persuasion, Ken Waltz was an intellectual force that had to be reckoned with.

He cared about writing style, and came down hard on his students, asking, “Who will bother to read you if you cannot make clear what you are thinking?” He would mark up one dissertation chapter and then write on it: “This is how it is done; do it right the next time.” His advice to many of his doctoral students became legend: “This thesis has problems and only you know how to fix them.” He believed that his task was to point out the problems; yours, to deal with them. Spoon feeding was not in Ken’s teaching vocabulary. Ken taught that the world of international politics was one of “self help”; if he was your thesis advisor that is the world you lived in.

Talking intellectual shop with Ken could be a forbidding experience. He was straightforward and direct, and had an uncanny instinct for the significant -- unlike so much scholarship today that seems to have an uncanny instinct for the insignificant. He could poke holes in most arguments with ease, had no patience for fluff and bombast, and yet was always kind, courteous, and constructive -- even as he shredded your work. He cared about brains and substance, not age and formal position or rank. He treated younger scholars he respected as his equal, not his subordinates.

Up to the very end of his life, he was a realist in the best sense of the term: he saw the world without blinders, but wanted, and acted through his writings and teaching, to make it a better and safer place by aiming for what was feasible, when feasible meant the second-best solution. He lived, wrote, and taught according to Aristotle’s dictum: the essence of political tragedy is to reject the good for the perfect. Ken aimed for the good because he knew the perfect was not in the cards.

He was a man with catholic interests and training. He read widely in literature, loved opera, majored in economics in college, and developed a keen interest in political theory during his political science doctoral work at Columbia. He came to international politics late in his graduate training, and became driven by the quest to provide some order to what was then a young and fairly chaotic field of study in the United States.

Ken wrote slowly and published less than any of the other leaders in the field. This was not because he had less to say, but because he worked with great care. As Tom Schelling said, “it takes him a long time to write because everything he does is read for a long time.” The way he wrote and the way he thought were related. His prose was tight and concise; there were relatively few words because each one counted. This is the way he approached theory, which he felt would be defeated if it tried to capture everything. The objective was to pare things down, to get at their essences, and to be willing to leave aside (or as he put it, “abstract from”) a great deal.

He was influenced in this approach by his early work in political theory and economics. The former taught him to concentrate on the matters of deepest and lasting significance. The latter reinforced this perspective and produced a taste for informal models that were deductive and rigorous. Thus although he never wrote down equations, both Bob Powell and Jim Fearon were his students and found his approach to theorizing exciting, enlightening, and congenial. Ken also liked economics because it kept one focused on incentives. This perspective put him at odds with most of IR research when he was starting his career in the 1950s, and led to the systemic argument he started in Man, the State, and War and developed in Theory of International Politics. It also underpinned his controversial views on proliferation. Individuals and states certainly differed, he believed, but in a setting that lacks the ability to make and enforce binding rules, the desire to survive produces enduring patterns, most obviously balance of power dynamics and war. While some leaders and some types of states may be somewhat more war-prone than others, the starting point for analysis must be the incentives that the environment sets for them (an environment that their behavior, in turn, helps shape).

Man, the State, and War gave the field these three “images” (or “levels of analysis,” as they became known later) and pointed to the weaknesses in the arguments for the causal role of human nature and regime type. But it did not explore why some systems were more likely to go to war than others. This was left for Theory of International Politics. Before he wrote it, most scholars had argued that bipolarity was less stable than multipolarity, and did so by pointing to the dangerous dynamics that characterized European politics before World War I. Ken’s breakthrough was to show that this system had actually been multipolar, and that its apparent bipolarity was the result of looking superficially at the alignments between the two camps rather than at distribution of power among five or six independent roughly equal states within them. The configuration after World War II, by contrast, was truly bipolar, and under this structure the superpowers could neither be dragged into war by their smaller allies, nor shirk their responsibilities in the hope that others would pick up the burden.

Ken’s focus on incentives and the primacy of security led him to believe that nuclear weapons made Soviet-American relations safer and that proliferation might similarly stabilize rivalries in other parts of the globe. Nuclear weapons were good for threatening retaliation but not much more, and so could freeze the status quo and bring peace. He certainly believed this argument, but it is worth noting that he also enjoyed the fact that few others did. He was, after all, a contrarian.

This trait was clear in Foreign Policy and Democratic Politics, his second and least-known book, which argued that contrary to what was believed on both sides of the Atlantic when he published it in 1967, the American political system was better designed to produce effective foreign policy than the British one. If this conclusion was somewhat self-congratulatory for the US, the same cannot be said for the rest of his work. It shows the falsity of the common claim that realism expects countries to be bellicose and acts like cheerleaders in urging them on. To the contrary, although Ken denied that his theory could explain states’ foreign policies, he argued that it indicated that during the Cold War and after, security did not require the US to fight distant wars, and he was a forthright opponent of the adventures in Vietnam and Iraq.

His family, friends, colleagues, and students mourn the passing of the man. Generations to come will continue to wrestle with his work.