When President George W. Bush addressed Congress and the nation on September 20, 2001, he made a case for a new kind of military response; not a targeted air strike on a single training facility or weapons bunker, but a wide-ranging global War on Terror.

“Our war on terror begins with al Qaeda, but it does not end there,” said Bush. “It will not end until every terrorist group of global reach has been found, stopped and defeated.”

When American troops invaded Afghanistan less than a month after September 11th, they were launching what became the longest sustained military campaign in U.S. history. The fight in Afghanistan had support from the American people and the backing of NATO allies to dismantle al Qaeda, crush the Taliban and kill Osama bin Laden, the murderous mastermind of the 9/11 attacks.

American support for the War on Terror became mixed as the campaign continued for years in an effort to target multiple terrorist cell and rogue regimes across the world. Thousands of American troops were killed in the first two decades of the War on Terror and many more returned home with physical and psychological wounds. Yet the ever-present shadow of 9/11, Jenkins says, kept U.S. troops on the ground in Afghanistan and elsewhere for nearly 20 years.

“What’s made it so difficult to back out of these conflicts is that fear that if we leave, we will leave behind a vacuum,” says Jenkins. “And in that vacuum the terrorists will return.”

One of the most disturbing aspects of the 9/11 attacks was that 19 al Qaeda hijackers were not only able to board commercial aircraft with crude weapons but also force their way into the cockpit. It was clear that 9/11 was both a failure of America’s intelligence apparatus to identify the attackers and a failure of airport security systems to stop them.

Even though there had already been a handful of high-profile hijackings and bombings of commercial planes, including the tragic 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, security was not a high priority for airlines before 9/11, says Jeffrey Price, a professor of aviation and aerospace science at Metropolitan State University and a noted aviation security expert.

“Airports had security departments and employees wore badges and they did screenings, but none of it was nearly at the level of what we do today,” says Price.

Before 9/11, people didn’t have to have a ticket to wander around the airport or wait at the gate. No one checked passengers' IDs before boarding the plane. And the only item people had to remove when passing through security was loose change from their pockets. Price says that most airports didn’t bother running background checks on their employees, and checked baggage was never scanned.

Just four days after the 9/11 attacks, a gunman in Mesa, Arizona went on a shooting rampage. First, he shot and killed Balbir Singh Sodhi, a gas station owner of Indian descent. Sodhi was Sikh, so he wore a turban. The gunman assumed he was Muslim. Minutes later, the gunman shot at another gas station clerk of Lebanese descent, but missed, and then shot through the windows of an Afghan-American family.

Even as politicians and law enforcement repeatedly stated that Islam was a peaceful religion whose true teachings had been twisted by terrorist extremists, many people in America and around the world still equated the 9/11 attacks with Islam and sought vengeance on anyone that even looked Muslim.

In the year 2000, there were only 12 anti-Muslim assaults reported to the FBI. In 2001, that number skyrocketed to 93. As civil liberties organizations criticized the TSA and law enforcement for the racial profiling of Arab and Muslim men, hate crimes against Muslims persisted. Statistics from the FBI showed there were 91 reported aggravated or simple assaults motivated by anti-Muslim bias in 2015 and in 2016, that figure surpassed 2001 numbers, reaching 127.

The Patriot Act was passed just six weeks after 9/11 as lawmakers scrambled to fix the intelligence failures that allowed known terrorists to enter the United States and execute the deadliest plot in American history. The controversial act authorized sweeping changes in how domestic intelligence agencies like the FBI conducted surveillance. Long-standing rules meant to protect Americans from “unreasonable search and seizure” were loosened or thrown out in the name of national security.

The fear, again, was that the 9/11 attacks were just the beginning, and that more terrorist cells were active in American cities and awaiting orders to strike. In order to find these “terrorists among us,” Congress gave the FBI and NSA new abilities to collect and share data. For example, the Patriot Act gave intelligence agencies the power to search an individual’s library records and internet search history with little judicial oversight. Agents could search a home without notifying the owner, and wiretap a phone line without establishing probable cause.

While civil liberties groups fought back against what they saw as unconstitutional breaches of privacy under the Patriot Act, an even more controversial law was passed in 2008, the FISA Amendments Act. This law gave the NSA nearly unchecked authorities to eavesdrop on American phone calls, text messages and emails under the premise of targeting foreign nationals suspected of terrorism.

In the years since 9/11, Americans inspired by jihadist ideology have killed 107 people in domestic terrorist attacks (as of September 2020). Almost half those deaths occurred in one horrific shooting at the Pulse Nightclub in Orlando, but there have been no large-scale terrorist attacks on U.S. cities like the ones many believed would inevitably follow September 11th.

“If anyone had said this is what they expected the threat to be the day after 9/11, they probably would have been laughed at,” says Sterman. “That would have seemed hopelessly naive.”

The security measures put in place after 9/11 appear to have foiled or discouraged another ambitious plot by foreign agents on American soil. But in the process, says Jenkins, the country has faced an “endless” War on Terror that has indelibly altered the fabric of American life.