A quarter-century has now passed since the devastating surprise terrorist attacks of September 11, 2001. This significant anniversary offers a valuable opportunity to reflect on and analyze the lessons of that day — and of everything that followed.

On a sunny September morning 25 years ago, al-Qaeda terrorists hijacked four civilian aircraft that had departed from East Coast airports and turned them into extraordinarily lethal suicide weapons of mass murder. Their weapons were box cutters, which could pass through security monitors — or, if detected, were permitted at the time. That vulnerability helps explain why the attacks reshaped aviation security worldwide: the United States created the Transportation Security Administration in November 2001, federalizing airport screening, and cockpits were reinforced industry-wide to prevent similar hijackings.

Two of the planes slammed into the Twin Towers of the original World Trade Center in New York City, inflicting structural damage severe enough to bring both towers down. A third plane crashed into the Pentagon, causing extensive damage and fires. The fourth aircraft, likely headed for the Capitol or the White House, never reached its target. Its courageous passengers, aware of what was unfolding, attempted to retake control of the plane. It plummeted into a field in Pennsylvania, killing everyone on board. In the years since, first responders and recovery workers at the sites have suffered lasting health consequences, and Congress has repeatedly addressed those harms through legislation, including the James Zadroga 9/11 Health and Compensation Act and its later extensions.

The death toll from these partially successful atrocities was high. A total of 2,753 people were killed in the World Trade Center attacks, with some seen jumping from the burning structures as a preferred alternative form of death. Adding the deaths at the Pentagon and the crash in Shanksville, Pennsylvania, the total reached 2,977. The victims came from more than 90 countries, underscoring that the attacks struck not only the United States but an international community concentrated in New York's financial district.

Counting the injured is more difficult, given the dense urban populations involved, the delayed onset of some injuries, and the general chaos and confusion that followed the attacks. Approximately 10,000 people in New York were treated immediately for serious injuries.

Individual and collective heroism defined the response to the unprecedented attacks — beginning with, but extending well beyond, the passengers over Pennsylvania who faced almost certain death yet decided, immediately and collectively, to act so as to at least defeat the terrorists. Hundreds of New York firefighters and dozens of police officers died in the towers' collapse while carrying out evacuations — losses that remain among the deadliest days in the history of both departments.

New York Mayor Rudy Giuliani rose to the challenge on this occasion, showing impeccable judgment and calm courage that reassured a public suffering from collective shock. Similarly, Defense Secretary Donald Rumsfeld, at work in the Pentagon when it was struck, immediately headed toward the devastation and pitched in to rescue and help survivors.

President George W. Bush and his administration initially reacted effectively and successfully to the shocking attacks. Bush was visiting a school in Florida when the attacks occurred. The trip was immediately interrupted; he boarded Air Force One and wisely flew to secure military installations in Louisiana and Nebraska rather than returning directly to Washington, D.C. Phone calls en route reconfirmed the influence of Vice President Dick Cheney and National Security Adviser Condoleezza Rice. National disaster relief and rescue personnel were dispatched to New York City and the Pentagon.

In a particularly effective move, Bush visited Ground Zero in Manhattan, demonstrating personal commitment. Standing atop rubble and debris, his arm around the shoulders of a firefighter, he spoke emotionally and effectively of prayer, patriotism and solidarity.

There was, at first, international solidarity as well. The U.N. Security Council supported the U.S.-led invasion of Afghanistan, where al-Qaeda had sheltered. Immediately after the attacks, French military aircraft patrolled the skies over North America — the first-ever implementation of Article 5 of the NATO Treaty, which stipulates that an attack on one member is an attack on all.

Then came a blurred expansion of the response. Bush declared an open-ended "war on terror," and the United States, joined by some allies, invaded Iraq, resulting in continuing strategic instability. The domestic front expanded as well: the USA PATRIOT Act, passed in October 2001, broadened government surveillance powers, and the Department of Homeland Security was created in 2002 as the largest government reorganization since the Cold War era. Two decades later, the U.S. withdrawal from Afghanistan in 2021 ended the longest war in American history, leaving open questions about the durability of the interventions launched in response to 9/11.

Arthur I. Cyr (acyr@carthage.edu) is the author of "After the Cold War – American Foreign Policy, Europe and Asia" (NYU Press and Palgrave/Macmillan).