U.S. soldiers and Iraqi civilians pull down a statue of Saddam Hussein in Baghdad on April 9, 2003. (Jerome Delay/Associated Press)

Each year, the anniversary of Sept. 11, 2001, brings a torrent of reflections on the attacks, Osama bin Laden and the future of terrorism. But the 2003 U.S. invasion of Iraq, justified in part by alleged links to 9/11, has had a far greater global impact. The invasion’s unintended consequences continue to reverberate today, setting the stage for profound changes in global, regional and local security dynamics. Many of the most important developments in the Middle East in the past 16 years trace their roots in blowback from the Iraq War rather than 9/11.

Political disintegration and sectarian war in Iraq

Newly available secret Baath Party documents allow us to know now, more than ever before, what was happening in Iraq in the run-up to 9/11.

In the summer of 2001, Saddam Hussein’s Baathist regime was preoccupied with managing public discontent stemming from economic sanctions, persistent power outages, the rising cost of vegetables and rumors about counterfeit bills in the economy. On the security front, the regime was seeking volunteers for a paramilitary brigade to support the second Palestinian intifada. Little did Hussein and his fellow Baathists know that the 9/11 attacks would set into motion a chain of events that would completely change Iraq — from a repressively ruthless, yet bureaucratically competent, regime to a near-failed state riven by sectarianism.

After the invasion, the U.S. de-Baathification purged the government of Baath Party members and disbanded the army, creating a power vacuum. Abu Musab al-Zarqawi, an independent jihadist from Jordan, had been waiting for just such an opportunity to draw out sectarian identity. In an undated secret communication with Osama bin Laden, Zarqawi advocated for attacks on Shiite religious, political and military targets to provoke a civil conflict. And Zarqawi largely achieved his aim. By 2006, Iraq was ravaged by a full-blown sectarian war that had left hundreds of thousands of Iraqis dead.

Sunni activists began to argue publicly that Iranian-sponsored Shiite militias were engaged in a genocidal campaign against Sunnis. The ceding of political space to Sunni Islamic extremists — long suppressed under the Baathists — set the stage for the rise of the Islamic State in the western governorates of Iraq and its expansion into Syria as civil war erupted there.

The emergence of the Islamic State

Although Zarqawi was killed by an American bomb in 2006, his successors continued intensifying sectarian attacks and renamed his organization the Islamic State of Iraq (ISI), no longer an arm of global al-Qaeda. By November 2006, the Islamic State’s Ministry of Media in Anbar province called on Sunni communities around the world to stand beside their sectarian brethren to support ISI.

The surge in coalition military forces as well as a Sunni backlash against the brutality of ISI weakened it considerably before the U.S. withdrawal in 2011. However, the Iraqi state’s inability to maintain security and the outbreak of civil war in Syria created new opportunities for the group. The Islamic State moved quickly in 2014 to conquer a surprisingly large swath of territory in Iraq and Syria, including the city of Mosul, and declared a caliphate under ISI leader Abu Bakr al-Baghdadi.

Arab popular mobilization galvanized

The unpopularity of the invasion of Iraq galvanized popular opposition movements and laid the foundation for the Arab uprisings of 2011. Egyptian anti-American protests in the wake of the invasion were unlike any witnessed since the 1970s. Because of the relatively weak justification for the invasion, Egyptian President Hosni Mubarak was forced to allow the demonstrations, including a day-long occupation of Tahrir Square, to dodge criticism about his own U.S. ties.

These protests opened the door to new forms of public engagement. While we cannot draw a direct line from antiwar protests to the Arab uprisings of 2011, the U.S. invasion of Iraq ushered in a new era of street protest across the Middle East.

Loss of U.S. moral authority

The United States lost much of its moral authority in the region and in the world as a result of widely publicized abuses at Abu Ghraib prison and the ethically and legally questionable detention of prisoners at Guantanamo Bay.

The Iraq War and its aftermath boosted anti-American sentiments throughout the Middle East, providing powerful ammunition to political entrepreneurs who sought to capitalize on U.S. missteps. Widespread negative perceptions of the United States continued to resurface, with deadly consequences. In September 2012, protests associated with the controversial “Innocence of Muslims” film led to attacks on U.S. diplomatic outposts across the Muslim world.

Empowering Iran

The regime change in Iraq also created a new regional ally for Iran and intensified the rivalry between Iran and the major Sunni powers of the Middle East. This destabilizing shift in the regional balance of power is playing out in military interventions in the civil wars in Yemen and in Syria. The United States finds itself in the awkward position of joining Iran in opposing Sunni extremists in Syria, while denouncing Iran on almost every other front — particularly its support for Syrian President Bashar al-Assad, suspected nuclear ambitions and sponsorship of Hezbollah.

Fear of Iran is sharpened by apprehensions about the influence of Iran’s Revolutionary Guard Corps (IRGC), especially its Quds Force, which has assisted pro-Iranian armed groups in Iraq, Syria, the wider Middle East and beyond — including a plot to assassinate the Saudi ambassador to the United States. The IRGC is also a powerful and repressive force in internal Iranian politics.

A feedback loop of transnational jihad

The immediate American military response to 9/11 elevated terrorism to the top of the security agenda and set back al-Qaeda. But military intervention also provided new impetus for jihadism. With the intensification of drone strikes on targets across a variety of Muslim-majority countries, many Muslims feel victimized, reinforcing the Islamic State’s messaging about the need to protect Muslim communities from Western aggression. Collectively, jihadist ambitions gained credibility and legitimacy, inspiring further attacks on Western targets. None was on the scale of 9/11, but they left hundreds dead and injured in cities like Madrid, London and Paris.

The perseverance of this diffuse but compelling ideological trend in transnational terrorism is both unusual and disturbing.

Transnational jihadist terrorism is also increasingly intermingled with civil war across the globe. The same actors are both civil war rebels and conspiratorial terrorists, and civil wars have attracted thousands of volunteer foreign fighters. The arrival of outside actors on the scene has “Islamicized” otherwise secular wars. Al-Qaeda has proved particularly adept at exploiting local grievances.

Perhaps one of the most enduring legacies of 9/11 is that terrorism provokes military intervention, which in turn intensifies civil conflict and encourages more terrorism and more intervention. Yet such an outcome was not inevitable. Had the United States responded differently to 9/11, today’s Middle East would be entirely different.

Lisa Blaydes is an associate professor of political science and a senior fellow at the Freeman Spogli Institute for International Studies at Stanford University.

Martha Crenshaw is a senior fellow at the Freeman Spogli Institute for International Studies.