I feel I should add my two cents to the torrent of IR-blogospheric comment on the late Kenneth Waltz, if only to justify my existence as a blogger. I never met Waltz[*] but like virtually every student of international relations I have read his two key books (not the third one). This post is basically a spur-of-the-moment thing, not the product of sustained thought, and it should be read with that in mind.
Man, the State and War (1959), hereafter MSW, and Theory of International Politics (1979), hereafter TIP, are rather different kinds of books, even if they both endorse a "structural" view of international politics. MSW is an analysis of what Great Thinkers in the (mostly) Western tradition have said about the causes of war. The book famously sorts these writers into three camps: those who locate the causes of war in human nature ("the first image"), in the characteristics of individual states ("the second image"), or in the 'anarchical' (meaning, essentially, no-world-government) structure of the international system ("the third image"). Waltz concludes that the third-image view is the most convincing, famously declaring that (to paraphrase him) wars occur because there is nothing to prevent them.
MSW is a very perceptive book that treats a subject of intrinsic interest, and for that reason it will continue to be read. But apart from the fact that (some) students are still required to read it, I doubt it exercises all that much influence over the field today. Few Ph.D. candidates in political science who "do" International Relations will write a dissertation these days about what this or that Great Thinker has said about the causes of war. Such dissertations are still written but I think they are rare, especially in the United States. Someone who is interested in both political/social theory and international relations is more likely nowadays to do what might be called critical disciplinary history (which is, for example, what Daniel Levine's book Recovering International Relations is, or so I gather from hearing him speak about it on one occasion before it was published). Disciplinary history is, obviously, about the development of the discipline or the field of International Relations; it is rather inward-focused. Waltz's MSW is not disciplinary history in this sense. That is not at all to criticize MSW, merely to note the difference.
The other aspect of MSW perhaps worth mentioning is that the phenomenon with which it was concerned, namely traditional interstate war, is now increasingly rare. When MSW was published in 1959, the Korean War had ended only six years before, World War II only fourteen years before. The Cold War was in full swing and there was no guarantee that it would not become hot. Today, although wars are, unfortunately, still with us, traditional interstate wars have become unusual events (the last really big one was the Iran-Iraq war of the 1980s), and wars directly between two or more great powers are unheard of. Uncovering the causes of interstate war had an urgency in the 1950s which, at least arguably, it no longer has. The 'hot topic' now is civil war, as will be apparent to anyone who glances through the list of political science dissertations completed in 2012 (in the U.S.) that was recently published in PS. I am not saying MSW is passé or no longer relevant; it will always be 'relevant' because its subject is of intrinsic interest, meaning it has inherent interest regardless of what is going on in the world, just as the study of history has intrinsic interest regardless of what is going on in the world. I'm simply observing that the focus of attention in the field seems to have shifted to other things.
Turning now to Theory of International Politics. The reverberations of TIP are still being felt in the field in a much more direct way than those of MSW. The emphasis in TIP on the "shaping" influence of structure -- meaning, in essence, the distribution of capabilities (power) among states under anarchy -- is still a starting point for many, though by no means all, scholars of international politics. However, TIP has come under several different kinds of criticism since it was published -- in fact, too many to catalog exhaustively here. A sampling: Ruggie charged that Waltz in TIP ignored questions of historical change and transition between different kinds of state system, something the English School had always been more attuned to, while Wendt criticized Waltz for not seeing that 'power' and 'interest' are mostly made up of ideas and that the effects of power accordingly depend on the distribution of ideas in the system. Other writers threw doubt on the notion of 'anarchy' and the states-under-anarchy model. And finally (for the non-exhaustive purposes of this post), many have criticized what is perhaps the central substantive proposition of TIP, namely the argument that balances of power recurrently form and that, in Waltz's words, "if there is any distinctively political theory of international politics, balance-of-power theory is it." The other major aspect of TIP was Waltz's views on what 'theory' is; his epistemological-methodological position continues to be both influential and controversial, but I will pass over it here.
Despite all the criticisms, TIP will remain required reading for students, if only so that they can follow the flood of critiques it unleashed. The appearance as recently as 2011 of a collection of essays about Waltz's work, Realism and World Politics (ed. Ken Booth), suggests that Waltz's writings will continue to generate interest and discussion for quite some time to come and will continue to be seen as canonical works in the field.
(Note: Post edited slightly after initial posting.)
Added later: Waltz's 1988 APSA presidential address, "Nuclear Myths and Political Realities," as published in the Sept. 1990 issue of APSR, is available here (pdf).
*I did hear him speak on one occasion but that doesn't amount to 'meeting'.