Victor Davis Hanson is a classicist and historian at the Hoover Institution, Stanford University, and the author, most recently, of The End of Sparta, a novel about ancient freedom.

Two terrible September days sum up the first decade of the new American millennium.

The first, of course, was Sept. 11, 2001. Osama bin Laden’s suicide terrorists that morning hit the Pentagon, knocked down the World Trade Center, killed 3,000 Americans, and left in their wake 16 acres of ash in Manhattan and $1 trillion in economic losses. Two invasions, into Afghanistan and Iraq, followed — along with a more nebulous third "war on terror" against Islamic radicalism generally.

America was soon torn apart over both the causes and the proper reaction to the attacks. The Left often cited America’s foreign interventions and Middle East policies as provocations. And it soon bitterly opposed the war in Iraq, and even more adamantly decried the antiterrorism protocols that followed 9/11.

The Right countered that only unwarranted hatred of the U.S. prompted the carnage. The best way, then, to prevent more Islamic terrorism was to go on the offensive abroad against regimes that sponsored terrorism, whether the Taliban or Saddam Hussein. New security protocols and laws at home were likewise needed to prevent another major terrorist onslaught.

But a decade later, the unforeseen had happened...