Stephen M. Walt, the Robert and Renée Belfer professor of international affairs at Harvard University's Kennedy School of Government and a contributing editor at Foreign Policy, is the author of Taming American Power: The Global Response to U.S. Primacy and, with co-author John J. Mearsheimer, The Israel Lobby. He blogs at walt.foreignpolicy.com.

What's the most powerful political force in the world? Some of you might say it's the bond market. Others might nominate the resurgence of religion or the advance of democracy or human rights. Or maybe it's digital technology, as symbolized by the Internet and all that comes with it. Or perhaps you think it's nuclear weapons and the manifold effects they have had on how states think about security and the use of force.

Those are all worthy nominees (no doubt readers here will have their own favorites), but my personal choice for the Strongest Force in the World would be nationalism. The belief that humanity is comprised of many different cultures -- i.e., groups that share a common language, symbols, and a narrative about their past (invariably self-serving and full of myths) -- and that those groups ought to have their own state has been an overwhelmingly powerful force in the world over the past two centuries. 

It was nationalism that cemented most of the European powers in the modern era, turning them from dynastic states into nation-states, and it was the spread of nationalist ideology that helped destroy the British, French, Ottoman, Dutch, Portuguese, Austro-Hungarian, and Russian/Soviet empires. Nationalism is the main reason the United Nations had fifty-one members immediately after its founding in 1945 and has nearly 200 members today. It is why the Zionists wanted a state for the Jewish people and why Palestinians want a state of their own today. It is what enabled the Vietnamese to defeat both the French and the American armies during the Cold War. It is also why Kurds and Chechens still aspire to statehood; why Scots have pressed for greater autonomy within the United Kingdom, and it is why we now have a Republic of South Sudan.

Understanding the power of nationalism also tells you a lot about what is happening today in the European Union...