Ikenberry and Mearsheimer are alright. It's too bad 1999 doesn't make the cut - there were great books from then, esp. Wendt's STIP.

Some others to consider:
-- Ken Schultz, Democracy and Coercive Diplomacy (2001)
-- Jeff Taliaferro, Balancing Risks (2004)
-- Keir Lieber, War and the Engineers (2005)
-- Daryl Press, Calculating Credibility (2005)
-- Andrew Kydd, Trust and Mistrust in International Relations (2005)
-- Patrick Thaddeus Jackson, Civilizing the Enemy (2006)
-- Lene Hansen, Security as Practice (2006)
-- Stephen Brooks, Producing Security (2007)
-- Stephen Brooks and Bill Wohlforth, World Out of Balance (2008)
-- David Edelstein, Occupational Hazards (2008)

1. Mearsheimer's Tragedy of Great Power Politics (2001). We all know: offensive realism and so forth.

2. Biddle's Military Power (2004). The book contributes dramatically to the Offense-Defense Theory Debate showing, in my view very convincingly, that force deployment (or tactics, or the human factor) matters more than the hardware in combat. In this sense, it is superior to Liber's War and the Engineers that is quote above.

3. Singer's Corporate Warriors (2003). It has opened a completely new field in Security Studies. No theory, but several theoretical reflections. And the field is going to be massively investigate in the future: just look how many PhD's are doing reseach on private companies and war.

4. Layne's The Peace of Illusions (2006). This book shows first the strenght of Neoclassical Realism. Secondly, it shows how the Bush era is more consistent with American foreign policy than we thought.

5. Gholz's and Dombrowski's Buying Military Transformation. Above it is quoted Brook's 2005 book. Well, Gholz and Caverley have shown that the work has major pitfall (SecStudies, Nov 2007). Gholz and Dombrowki make instead a very sophisticate argument about the future of the american defense industry and about the link between government, strategic environment and industry.

Forgive my Security Studies bias.

Posner and Goldsmith's 2005 The Limits of International Law. These two guys are lawyers, but their point concerns us deeply. During the 1990s, we heard all these discussions about the constitutive effects of norms, socialization and so forth. Their book dramatically refutes these views showing that international law is applied and respected only to extent that it serves states' interests.
This was a highly needed book.

Out of the books already listed here, I think I'd go with one of those. I really like the idea of going with a methodology text. But Kydd's Trust and Mistrust in International Relations should definitely be in the mix.

As to books that haven't been mentioned yet I'd throw in David Welch's Painful Choices and Ned Lebow's The Tragic Vision of Politics.

As to other books that have been mentioned, I quite like both Layne and Ikenberry but I don't know that I'd given them this prize. I certainly wouldn't give it to Mearsheimer (having a lot of holes in your book that ensure it'll be a subject for criticism for years ... I don't see how that makes it "best"). And I've never read a Farewell to Alms. I should do that at some point.

I'm sensing a bit of a realist bias here. . . .

It depends on how you define "best". For example, Mearsheimer's book is the culmination of debates in the '80s and '90s that most people think are over. . and not because realism "won".

If you want the two books that probably have influenced more debates in actual political science journals and the way more graduate students who didn't go to the University of Chicago think about international politics, two books no one has mentioned are:

1. Bdm et al. Logic of Political Survival

2. Reiter and Stam. Democracies at War

Both have had a huge impact on the *field* of international relations, as in how people think about and debate about these topics.

p.s. I think it is good that people moved on from the "high theory" debates to actually trying to test things and move things forward, empirically. That doesn't mean there weren't great books in this decade, just that their "form" will look different than the best books of previous decades.

Some good IR books in the last ten years:

1. R. Harrison Wagner's (2007) War and the State. Although many have noted that Mearsheimer's theory is not logically consistent, and therefore completely worthless, Wagner's treatment is the fairest and most developed.

2. Bruce Bueno de Mesquita, Alastair Smith, Randolph Siverson, and James Morrow. 2003. The Logic of Political Survival. Is there a better theory of foreign policy and international relations out there? Their winning coalition theory explains a lot.

3. Michael Tomz. 2007. Reputation and International Cooperation. Among the various books on cooperation, this one is the most theoretically cogent and empirically rigorous.

The best. The most important. The most needed. The most courageous. The most honest.

Does anyone else find both the list and the comments really depressing?

Tell me in the abstract what characteristics a great book should have.

Now tell me whether these books have those characteristics.

P.S. I like Wedman's P.S. that the proper criteria for a great book may be changing.

I can't be certain how the decision was made on which book was best as the criteria is a bit vague. Can it be between only two nations, or does it require the moves of multiple nations? Is it only on recent history or something further back?

Although it's been stated in other works, Fareed Zakaria's Post-American World might be a strong contender.

On Iraq and American politics between 2003 and 2006 (or at least what an Iraqi politician sees of it) Ali A. Allawi's The Occupation of Iraq might be a good choice.

For the United States, Pakistan, Afghanistan, and the USSR in the 1980s perhaps Steve Coll's Ghost Wars.

For Central Asia over 2001 to 2007 Ahmed Rashid's Descent Into Chaos gives a good idea of Pakistan, Afghanistan, and the U.S.

Without a doubt it should be Dan Deudney's Bounding Power. If you put in the time to understand his funky language, it will blow your mind and change the way you think more than anything has since Waltz and Wendt.

If we've got to give it to one of the more mainstreamers, though, it should be Ikenberry.

Mearsheimer's book shouldn't even be in the conversation.

I have to ask: What exactly is it in a logically incoherent book like "The Tragedy of Great Power Politics" that "linger[s] the most in the cerebral cortex"? That there's no there there in neo-realism is a point Powell made pretty well in nearly all of his writings throughout the 90's. Maybe he was too polite about it, so no one noticed that the party was over. (Grieco, for one, seemed to be blissfully unaware that he had had his posterior handed to him by Powell in the absurd absolute v relative gains debate spawned by Keohane and Axelrod.) Why did we have to go through all of this again in 2001? We need to stop pretending that there is a debate here. "Parsimony" is only an asset if the argument makes sense. Let's try to limit ourselves to considering explanations that actually, you know, explain things.