America After 9/11: Power and Its Limits

On the morning of September 11, 2001, the most powerful country in the world — the sole superpower of the unipolar moment, the country that had just won the Cold War and watched its most serious rival collapse — was attacked by nineteen men armed with box cutters. The attacks killed nearly three thousand people, destroyed the World Trade Center, and damaged the Pentagon. They also triggered a response that would define American foreign policy for two decades and cost, by various estimates, trillions of dollars and hundreds of thousands of lives.

The initial response — the invasion of Afghanistan in October 2001 to dismantle the al-Qaeda infrastructure and remove the Taliban government that harboured it — was broadly understood and internationally supported. Within weeks, the Taliban government had collapsed. Bin Laden had escaped into the mountains. What came next was a nation-building project for which the United States was ill-suited, in a country that had exhausted several previous imperial ambitions.

The invasion of Iraq in March 2003 was more consequential and more contested. The stated case for war — that Iraq possessed weapons of mass destruction — proved false. The underlying reasoning was more complicated, involving concerns about regional stability, the character of Saddam Hussein's regime, and a broader theory about remaking the Middle East. The war succeeded in removing Saddam but produced a power vacuum that generated years of insurgency, the rise of ISIS, and a regional destabilization whose effects are still being felt.

The wars in Afghanistan and Iraq revealed something that the planners had underestimated: the gap between military supremacy and political control. The United States could defeat any conventional military force on earth with devastating efficiency. It could not, it turned out, translate military victory into durable political outcomes in societies it did not understand, in conflicts where the opponent could simply outlast it by accepting greater casualties over a longer time horizon.

At home, the post-9/11 period produced a significant expansion of the surveillance and national security state. The Patriot Act dramatically expanded government surveillance powers. The Department of Homeland Security was created. The NSA's collection programs, revealed by Edward Snowden in 2013, turned out to be far more extensive than most Americans had realized. The debates about the balance between security and civil liberties, between executive power and constitutional constraints, were among the most important of the period and remain unresolved.

The two decades after 9/11 ended the easy confidence of the unipolar moment. America was still the most powerful country in the world by every conventional measure. But it had demonstrated that power has limits, that military supremacy is not the same as strategic success, and that the enormous costs of two long wars had consequences at home as well as abroad. The country that emerged from those two decades was more uncertain about its role in the world than it had been when they began.