The post–September 11 anti-war movement is an anti-war social movement that emerged after the September 11 terrorist attacks in response to the war on terror.

Background On September 11, 2001 a series of coordinated terrorist attacks against the United States killed approximately 3000 people. These attacks appear to have been carried out by a small group of individuals who formed part of the al-Qaida network: Islamists without formal backing from any state (though there were and are suspicions that Al-Qaida was aided and funded by several Arab/Muslim countries). Following them U.S. president George W. Bush declared a campaign with the stated aim of defeating terrorism which he called the "war on terrorism".

Although which of his programs constitute part of this "war" has never been formally articulated, the term appears to embrace at least two major Bush administration initiatives: a set of changes to U.S. criminal law and immigration law (most notably through the USA PATRIOT Act) and the 2001 invasion of Afghanistan and the 2003 invasion of Iraq. The term may also embrace such related matters as the creation of a Department of Homeland Security.

Many of those on the left and others who would come to oppose the war on terrorism did not believe that it was really a response to the terrorist attacks. They point to the Project for the New American Century as proof that Bush was merely using the atrocities as an excuse to put the Imperialist plans of the neoconservatives into action. They also point to what they perceive as the ineffectiveness of Bush's strategy for actually reducing terrorism and the lack of any link between Saddam Hussein and al-Qaida.

Immediate reaction to the attacks The immediate, worldwide reaction to the attacks was widely described at the time as "shock". No national government claimed responsibility or connection to the attacks. Indeed, the governments most associated with Islamism sought to distance themselves from the attacks. Wakeel Ahmed Mutawakel, the foreign minister of Afghanistan's then-ruling Taliban government, declared, "We denounce this terrorist attack, whoever is behind it." Mohammad Khatami, the Iranian president, said he felt "deep regret and sympathy with the victims." Shaykh Abdul Aziz al-Ashaikh, Grand Mufti of Saudi Arabia and Chairman of the Senior Ulama, said, "Hijacking planes, terrorizing innocent people and shedding blood constitute a form of injustice that can not be tolerated by Islam, which views them as gross crimes and sinful acts." Palestinian President Yasser Arafat said, "We completely condemn this serious operation... We were completely shocked..." Some, though, considered those reactions as hypocrisy, since several Arab and Muslim states encourage anti-Americanism and a number of newspapers in the Arab world—for example the Islamist opposition press in Egypt—openly celebrated the September 11 attacks. Moreover, states like Iran and Syria were known for long-year funding of terrorist networks such as Hezbollah, Hamas and Islamic Jihad. Also, Al-Qaida training camps were operating undisturbed in Afghanistan and the organization held bank accounts in Saudi Arabia.

On the left, condemnation of the attacks was equally general, although often including (even in the days immediately after the attack) condemnation of ostensibly related aspects of U.S. policies. Noam Chomsky's statement in the immediate wake of the attacks begins by condemning this "major atrocit[y]" and "horrendous crime", but also by contextualizing it in terms of the Clinton-era U.S. attack on the Al-Shifa pharmaceutical factory and prefiguring what would be a widespread concern for the left worldwide: "...the crime is a gift to the hard jingoist right, those who hope to use force to control their domains." Similarly, from Vijay Prashad, "The attacks must be condemned without reservation. But we must be certain to recognize that these are probably the work of frustrated and alienated human beings hemmed in by forces that are anonymous and that could only be embodied by these structures." Martin Woollacott, writing in The Guardian, called the attacks, "above all a stupendous crime", but also wrote, "America's best defence against terrorism originating from abroad remains the existence of governments and societies more or less satisfied with American even-handedness on issues which are important to them. Plainly, this is furthest from the case in the Muslim world."

Elected officials generally identified as being on the U.S. "left" also joined in strongly condemning the attacks, in this case almost universally without pointing out a context. For example, the day after the attack, Senator Edward Kennedy described the attack as "vicious and horrifying... acts of unspeakable cruelty... a massive tragedy for America", and commended President Bush for "his strong statement... about finding and punishing the perpetrators of this atrocity." Three days after the attacks, Congress passed a resolution authorizing President Bush to use force against "those responsible". The Senate voted 98–0, the House 420–1, with only Barbara Lee (D-California) dissenting. In a lengthy interview explaining her dissent, Lee pointed to her professional training as a social worker and remarked, "Right now, we're dealing with recovery, and we're dealing with mourning, and there's no way... [we should]... deal with decisions that could escalate violence and spiral out of control."

An anti-war movement forms Within days of the September 11 events, it was widely agreed that the attacks were carried out by al-Qaida, although , al-Qaida responsibility for the attacks may have been a minority view in majority-Muslim countries, though not among Muslims in the U.S. A small segment of the population also calls this belief into question. A much larger segment (though still a minority) of the left (both in the U.S. and elsewhere) concurred with the clear majority of Muslims that a military attack on Afghanistan was not the correct answer to the September 11 events. This anti-war view was even more widespread among both leftists and Muslims with respect to the later attack on Iraq.

The left was somewhat fragmented with respect to the invasion of Afghanistan. U.S. Representative Dennis Kucinich, who had opposed the Kosovo War and would soon oppose the invasion of Iraq, voted to authorize military action against Afghanistan, although he would later characterize it as a "disaster", a "nightmare", and "counterproductive". U.S. Representative Cynthia McKinney, speaking on September 24, acknowledged that "We must find and hold accountable all those who perpetrated those most terrible crimes against our nation and its people", but denounced what she saw as impending "suspensions of fundamental civil liberties" and said that she was "greatly concerned that we are about to engage in an extremely hazardous military campaign of unknown duration, with unrealistic objectives and perhaps even ultimately harmful long-term consequences for our nation", adding, "Already there is growing disquiet in the Muslim world that the U.S. is poised to turn its terrorist campaign into a war against Islam." Indian leftist writer Arundhati Roy, writing on September 29, strongly condemned both the attackers who had "blown a hole in the world as we knew it" and Bush for reacting by going to war against Afghanistan: "President Bush's ultimatum to the people of the world 'If you're not with us, you're against us' is a piece of presumptuous arrogance. It's not a choice that people want to, need to, or should have to make."

Within a few weeks after September 11, it became clear that two major prongs of the Bush administration's "war on terrorism" were to be a set of changes to U.S. criminal law and immigration law and an invasion of Afghanistan. An international anti-war movement began to arise; in the U.S. and other countries whose governments enacted legislation analogous to the PATRIOT act, it was equally a movement in protest of what were perceived on the left to be assaults on civil liberties and immigrant rights. This movement constituted a loose coalition of groups united in their opposition to U.S. military campaigns in the Middle East. Most prominent in the ranks of the movement were leftists, pacifists, and others with longtime associations with global peace movements, and Arabs and Muslims, including, but by no means limited to, Islamists. Most commentary focuses on the "anti-war movement" in the singular, although in some ways it could be argued that there is a plurality of different anti-war movements, who may not have anything in common with each other beyond their shared opposition to U.S. foreign policy.

The movement (or movements) included an enormous variety of groups and individuals that could not be categorised as "left" in any conventional terms, who had a variety of reasons for opposition to the invasion of Afghanistan and later of Iraq. In addition to the non-leftist Arabs and Muslims in the movement, there were also European nationalists uncomfortable with U.S. unilateralism (their numbers would greatly increase in the run-up to the invasion of Iraq). There was also an uneasy relationship with explicitly antisemitic groups who charged that the war was being waged on behalf of Israel, with the few, small right-wing anti-war groups, and with certain political fringe groups, such as the followers of Lyndon LaRouche. These latter groups sometimes participated in the same demonstrations with other opponents of the war, but seldom were actively involved in any of the same organized coalitions.

Almost no one denied the connection between Afghanistan's Taliban rulers and al-Qaida. However, various leftists opposed the Afghanistan invasion and the subsequent invasion of Iraq on grounds of pacifism, belief that the war was illegal under international law, opposition to perceived U.S. imperialism, disbelief (especially in the case of Iraq) in the sincerity of the U.S.'s stated war aims of counter-terrorism and the spread of political freedom, belief that the wars were motivated by neocolonialism and petroleum politics, and, in a few cases, denial of al-Qaida's responsibility for the September 11 attacks.

Another argument against the Afghanistan invasion was that war would bring unnecessary suffering on the people of Afghanistan and that it was not the most effective way to dislodge or isolate ‌that in fact it would inflate their importance and gain them recruits. Similarly, in the case of the later invasion of Iraq, some felt that Iraq was not a threat to the United States and that a preemptive attack was morally wrong though Saddaam Hussein was widely seen as a violent dictator. A number of people condemned the latter war because they doubted U.S. and other claims about Saddam's relation to al-Qaida, about his possession of weapons of mass destruction (WMDs), and about the effectiveness of war as a means of containing WMDs, or because of the lack of United Nations support for war.

Many Islamists and Arabs, and a few leftists, saw the military campaigns as battles in a religious ‌a ‌against Islam. This was the obverse of the ideas expressed, for example, by Samuel P. Huntington in The Clash of Civilizations and the Remaking of World Order. However this view was mostly rejected by those on the left, who generally rejected the doctrine that the world is divided into civilisations with fundamentally opposed cultures.