The September 11 attacks have had profound and wide-ranging political, economic, health and societal effects. They also reshaped global security policies and international relations, prompting lasting changes in how nations address terrorism and collective safety. It gave rise to the Global War on Terrorism, which included occupations of Afghanistan (2001–21) and Iraq (2003–11) and various other US-led interventions in the region.

The attacks elicited major rescue and recovery efforts in the affected areas, in particular at the World Trade Center site, which was rebuilt to include a memorial to the victims. Many of the hundreds of workers assisting in these efforts, as well as survivors and people living and working near the site, became seriously ill in the following years and decades due to exposure to toxins, which had spread throughout New York City as a result of the Twin Towers’ collapse. Mental health problems, especially post-traumatic stress disorder (PTSD) among ordinary citizens and first responders also increased as a result of the attacks.

Nationalism, solidarity and widespread support for first responders and the military pervaded the United States throughout the immediate aftermath of the attacks. Islamophobia, anti-Arab racism, and hate crimes and other discriminatory actions surged against members of ethnic and religious minorities in the US and other Western countries. The attacks cost billions of US dollars in damage and insurance claims, caused thousands of office workers to be displaced or lose their jobs, and dealt damage to the global economy. Cultural effects include millions of dollars worth in artwork being lost at the World Trade Center and the Pentagon, and media being adapted to remove any possible similarities to the attacks or references to the Twin Towers in the immediate aftermath.

Having occurred early in the twenty-first century, 9/11 is considered to be a turning point in the history of the US, which was in a relatively unchallenged position in global politics since the end of the Cold War a decade earlier. George W. Bush, who became US president in January 2001, dedicated much of his presidency to retaliation against the attacks and vast counterterrorism measures, in a series of interventions he dubbed the "war on terror". The US government established the United States Department of Homeland Security and increased military operations, surveillance, economic measures, and political pressure on groups that it accused of being terrorists, as well as increasing pressure on governments and countries it accused of sheltering them. October 2001 saw the first military action initiated by the US. Under this policy, NATO invaded Afghanistan to remove the Taliban regime (which harbored al-Qaeda) and capture al-Qaeda forces, invoking its Article 5 for the first time in its history. The US government also asserted that the Iraqi government was connected to 9/11.President Bush said "The battle of Iraq is one victory in a war on terrorism that began on September 11, 2001..." , and the Vice President Dick Cheney suggested that Iraq was involved in the September 11 attacks during a "Meet the Press" interview: Iraq is "the geographic base of the terrorists who had us under assault now for many years, but most especially on 9-11" (Knight-Ridder October 3, 2003, archived at ). Also, the US government has continued to maintain that the war on Iraq is critical to the American "War on Terrorism". "In the war on terrorism, Iraq is now the central front..." said President Bush on December 14, 2005. These efforts were started with near-universal support from Americans, but have since become the topic of strenuous debate. Critics point out that the Afghan conflict has contributed to the destabilization of neighboring Pakistan and Afghanistan has undergone a long war, culminating in the return of the Taliban in 2021.

Immediate Rescue and recovery/Post-9/11

Because of the events that took place on September 11, 2001, American society as a whole suffered dramatically. Recovery took years, and the economy declined drastically after the attacks. More than 1,500 first responders, ironworkers, engineers, heavy equipment operators, and other workers worked at Ground Zero to attempt to find survivors and clean up the wreckage. Cranes and bulldozers were brought in along with search and rescue dogs to locate survivors and bodies of the deceased, however, operations were hindered by the presence of approximately two feet of soot at the site, which obscured objects and bodies. In the immediate aftermath of the attacks, only 20 survivors were pulled alive from the rubble, although there were several human remains and belongings that were removed from the site. The day after the attack, then-mayor Rudy Giuliani told reporters that they were receiving mobile phone calls from people trapped in the debris. The task of removing debris and rubble continued well into 2002, with some 108,000 truckloads of 1.8 million tons of rubble removed by May 2002.

Hazards Outside of the general hazards due to fires, falling debris, heavy machinery, broken metal, and hazardous air conditions, there were also hidden concerns within the clean-up zone. The parking garage under the World Trade Center at the time of the attacks held nearly 2,000 automobiles; each held an estimated 5 gallons of gasoline, which could ignite and explode. Other concerns were around buried fuel tanks that were located on the site, and the 1.2 million rounds of ammunition that were housed at Building 6 for the use of the U.S. Customs Service.

US public reaction

Americans reacted to the terrorist attacks with a mix of shock, sadness, anger and fear. Following the September 11, 2001 attacks, George W. Bush's job approval rating soared to 86%. On September 20, 2001, the president spoke before the nation and a joint-session of Congress, regarding the events of that day, the intervening nine days of rescue and recovery efforts, and his intent in response to those events in going after the terrorists who orchestrated the attacks. In the speech, he characterized the speech itself as being akin to the President's customary State of the Union address. The attacks also had immediate and overwhelming effects upon the United States population. People began rallying around the popularized phrase, "United We Stand," in hopes of being resilient and keeping the American spirit alive in the face of a devastating attack. The majority of the US population rallied behind President Bush and the federal government in widespread support to the recovery and the expectant reaction to the attacks. The highly visible role played by Rudy Giuliani, the Mayor of New York City, won him high praise nationally and in New York City. He was named Person of the Year by TIME magazine for 2001, and at times had a higher profile in the US than President Bush.

Two major public reactions to the attacks were a surge of public expressions of patriotism not seen since World War II, marked most often by displays of the American flag; and an unprecedented level of respect, sympathy, and admiration for New York City and New Yorkers as a group by Americans in other parts of the United States. Some criticized this particular reaction, noting that not everyone who died was from New York City (for example, some of the passengers on the planes) and that the Arlington County, Virginia community also suffered in the attacks. Many people joined together to help the victims. Gratitude toward uniformed public-safety workers, and especially toward firefighters, was widely expressed in light of both the drama of the risks taken on the scene and the high death toll among the workers. Many people paid tribute to the police officers and firefighters who died during the attacks by wearing NYPD and FDNY hats. The number of casualties among the emergency service personnel was unprecedented.

Blood donations saw a surge in the weeks after 9/11. According to a report by the Journal of the American Medical Association, "...the number of blood donations in the weeks after September 11, 2001, attacks was markedly greater than in the corresponding weeks of 2000 (2.5 times greater in the first week after the attacks; 1.3–1.4 times greater in the second to fourth weeks after the attack)." At the Westminster Kennel Club Dog Show that took place in New York in February 2002, a tribute was paid to the search and rescue dogs who not only assisted in locating survivors and bodies from the rubble but were also inside the World Trade Center buildings before they collapsed.

Backlash and hate crimes In weeks following the attacks, there was a surge in incidents of harassment and hate crimes against South Asians, Middle Easterners, and anyone thought to be "Middle Eastern-looking"—particularly Sikhs, because Sikh males usually wear turbans, which are stereotypically and erroneously associated with Muslims by many Americans. Balbir Singh Sodhi, a Sikh man, was one of the first victims of this backlash; he was shot dead on September 15 at the gas station he owned in Mesa, Arizona. Mark Anthony Stroman, a white supremacist, killed two men and injured a third in a shooting spree beginning September 15 in Dallas, Texas. His victims, including Bangladeshi American Rais Bhuiyan, were all targeted because they looked "of Muslim descent". His motive for the killings was revenge for the 9/11 attacks. In New York City itself, the only death possibly related to post-9/11 hate violence officially recorded as a homicide was Henryk Siwiak, a Polish immigrant shot in Bedford-Stuyvesant, Brooklyn on September 11. His family theorized he may have been the victim of a hate crime in the wake of the attacks, since he was wearing camouflage clothing, had dark hair and spoke imperfect, heavily accented English. The case remains unsolved; police are open to the family's theory but have not classified the killing as a bias crime.

In many cities there were reports of vandalism against mosques and other Islamic institutions, including some cases of arson. In the year after the attack, anti-Muslim hate crimes jumped 1,600 percent. The FBI and other government agencies arrested and deported Arabs and Arab Americans in higher numbers after the attacks than before, often with limited or insufficient evidence to connect them to purported terrorism.

As memorials were set up all over NY and even in the 9/11 museum, Talat Hamdani's request to honor her son, Salman Hamdani, was denied.Ridgely Ochs (January 6, 2012). "Seeking son's honor for 9/11 first response". Newsday. Salman Hamdani was an off-duty EMT and an NYPD cadet who died as he went to the towers to help save others. Salman Hamdani was Pakistani-American and his mother, Talat Hamdani, received public assaults of his character as they believed because of their Muslim faith, he must have been involved with the attacks as he was missing. Talat was questioned and asked about the reasons Salman joined the NYPD, his travel experience and who he was in contact with. News outlets like the New York Post published a story, "Missing or Hiding? — Mystery of NYPD Cadet from Pakistan",William J. Gorta (October 12, 2001). "Missing – or Hiding? Mystery of NYPD Cadet from Pakistan". New York Post. and multiple "Wanted" posters that had pictures from his cadet days stating "Hold and detain. Notify: major squad case".Sharon Otterman (January 1, 2012). "Obscuring a Muslim Name, and an American's Sacrifice". The New York Times. His body was found a month after the event and he was post-humously declared a hero by Congress 45 days after the attack, although his name was originally placed under the list of non-first responders.Dean Schabner (June 29, 2005). "Debate Heats Up Over 'Sunsetting' Parts of Patriot Act". ABC News.