We watched the towers fall. All of us did—on screens in offices, in homes, in the common spaces where we gathered trying to understand what had just happened. Two planes, two buildings, nearly 3,000 dead in minutes. And something fundamental shifted in the American consciousness that day.

I want to walk you through what happened next: how a shocked nation moved from the rubble of Manhattan to the decision for military action. Because the path from September 11 to Afghanistan was not inevitable. It was constructed through specific decisions, made by specific people, in real time, often with incomplete information.

The morning itself is almost too familiar now to need retelling. At 8:46 a.m. Eastern time, American Airlines Flight 11 struck the North Tower of the World Trade Center. The immediate assumption in some quarters was an accident—a small plane in the fog. By 9:03 a.m., that illusion ended. United Airlines Flight 175 hit the South Tower on live television. The American people understood, in that moment, that they were under attack.

At the White House, President Bush was in a motorcade en route to an elementary school in Florida. His national security advisor, Condoleezza Rice, was in her office. Vice President Dick Cheney was initially in his own office before being rushed to the secure bunker beneath the White House—the Presidential Emergency Operations Center, or PEOC.

The first question was always: who did this? By late morning, the assumption was hardening around al-Qaeda. The group was not new to American counterterrorism. In 1998, al-Qaeda had bombed the U.S. embassies in Kenya and Tanzania, killing 224 people. In 2000, the USS Cole was attacked in the port of Aden by a suicide boat. But 9/11 was different in scale, sophistication, and intent. This was not a distant terrorist attack on a symbolic target. This was New York.

What was known about al-Qaeda before 9/11? The group, founded by Osama bin Laden in 1989, had been operating in Afghanistan under Taliban protection since 1996. The CIA had been tracking bin Laden and his network, and there were warnings in the summer of 2001 that an attack might be coming. The August 6 Presidential Daily Briefing titled "Bin Laden Determined to Strike in US" laid out the threat. But the specific nature of the attack, the use of commercial airliners as weapons, was not anticipated.

By evening on September 11, the National Security Council had convened. The decision architecture was becoming clear: this was not a law enforcement problem. This was war. And the enemy was al-Qaeda, operating from Afghanistan with the protection of the Taliban government.

Over the following days and weeks, the Bush administration moved with remarkable speed. On September 14, Congress passed the Authorization for Use of Military Force (AUMF), giving the President broad authority to wage war against al-Qaeda and associated organizations. The language was expansive: it authorized the use of "all necessary and appropriate force" against anyone the President determined planned, authorized, or aided the attacks. The vote was not close: 98 to 0 in the Senate, 420 to 1 in the House. The sole dissent came from Representative Barbara Lee of California.

The decision for Afghanistan followed naturally from this authorization. The Taliban government had harbored al-Qaeda. They had refused American demands to hand over bin Laden. From the administration's perspective, they were complicit, and therefore a legitimate military target. Rumsfeld's Pentagon began planning what would become Operation Enduring Freedom.

But there was a debate within the administration about the nature of the response. Some wanted a broad war against all terrorist organizations worldwide. Others, including Colin Powell at State, argued for a focused approach against the specific threat: al-Qaeda and the Taliban. The broader vision would win out eventually, but in September 2001, the target was clear.

The decision for war was made in the context of shock, grief, and something close to consensus. There was no significant political opposition. The media was largely supportive. And the enemy appeared clear: a foreign terrorist organization and the regime that harbored it.

What no one could predict in September 2001 was how long this war would last, how its objectives would shift, how the pursuit of al-Qaeda in Afghanistan would eventually lead to an invasion of Iraq, how counterterrorism would evolve into counterinsurgency, or how two decades later, the same Taliban that fell in 2001 would return to power.

But that is the story of what came after. This is the story of the decision itself: how shock became action, how grief became authority, and how a nation moved from the worst terrorist attack in its history to a global war that would define a generation.