Second, enforcing “liberal hegemony”—a grand strategy of promoting global democracy and peace underwritten by U.S. military power—is simply beyond America’s capabilities. As the wars in Iraq, Afghanistan, and, earlier, Vietnam showed, the United States does not have the military resources and political will necessary to impose friendly democratic regimes upon distant peoples. Nor, as all three of those wars also demonstrate, does it have the ability to utterly destroy its enemies forever. Nor, finally, can the U.S. ensure, militarily or otherwise, that no one anywhere gets nuclear weapons—after all, if it could, presumably Pakistan and North Korea wouldn't have them.

The effort to control and police the world through force of arms makes the United States less secure in numerous ways, Posen argues. It bleeds U.S. resources, both military and economic, while leaving the country less prepared to face immediate threats. The belief that America will act as the world’s policeman encourages some of its allies to skimp on their own defense spending, forcing the U.S. to undertake further costly investments it cannot afford in the long term. In its role as Liberal Hegemon, it also encourages aggression and risky behavior in states like Israel, which can put off peace deals and engage in provocative actions like settlement construction because of the elaborate pledges of support it has received from America.

Rather than imposing American will by force, Posen suggests that we could more fruitfully and practically engage the world in other ways. For instance, if the U.S. is concerned about genocide, we could join the International Criminal Court and support the prosecution of those who commit war crimes (including, though Posen does not say this, American officials, at whatever level, who condoned, or condone, torture.) If we want to save people, we could honor our commitments under international treaties and open our borders to refugees; as Posen says, we are “rich enough to receive many individuals in such dire straits.” We could also send aid to poorer countries to encourage them to receive refugees.

Posen makes a compelling argument. But he makes it almost entirely on realist grounds. He advocates a policy of restraint because it will make the U.S. stronger and more secure, not—or at least not primarily—because a policy of restraint is more ethical than the alternative. His humanitarian suggestions—joining the ICC, opening borders—are addendums to, rather than the essence of, his reasoning.

But liberal hegemony, the argument Posen is rebutting, isn’t just based on security interests. It’s also predicated on morality. For instance, the rationale for invading Iraq was not only that the United States needed to crush Saddam Hussein for its own safety. It was also that Saddam was uniquely evil and that it would be good for the people of Iraq, and for people around the world, if he were destroyed. Similarly, the continuing presence of U.S. troops in Afghanistan is justified not only on the basis of protecting America from al-Qaeda, but also on the grounds that the Taliban are hideously oppressive, especially to women, and that it is America’s responsibility to stop them from returning to power.