Black September (Arabic: أيلول الأسود, romanized: Aylūl al-ʾAswad), also known as the Jordanian Civil War, was an armed conflict between Jordan, led by King Hussein, and the Palestine Liberation Organization (PLO), led by chairman Yasser Arafat. The main phase of the fighting took place between 16 and 27 September 1970, though certain aspects of the conflict continued until 17 July 1971.

After the 1967 Six-Day War, Palestinian fedayeen guerrillas relocated to Jordan and stepped up their attacks against Israel and what had become the Israeli-occupied West Bank. They were headquartered at the Jordanian border town of Karameh, which Israel targeted during the Battle of Karameh in 1968, leading to a surge of Arab support for the fedayeen. The PLO's strength grew, and by early 1970, leftist groups within the PLO began calling for the overthrow of Jordan's Hashemite monarchy, leading to violent clashes in June 1970. Hussein hesitated to oust them from the country, but continued PLO activities in Jordan culminated in the Dawson's Field hijackings of 6 September 1970. This involved the Popular Front for the Liberation of Palestine (PFLP) seizing three civilian passenger flights and forcing their landing in the Jordanian city of Zarqa, where they took foreign nationals as hostages and blew up the planes in front of international press. Hussein saw this as the last straw and ordered the Jordanian Army to take action.

On 17 September 1970, the Jordanian Army surrounded cities with a significant PLO presence, including Amman and Irbid, and began targeting fedayeen posts that were operating from Palestinian refugee camps. The next day, 10,000 Syrian troops bearing Palestine Liberation Army (PLA) markings began an invasion by advancing towards Irbid, which the fedayeen had occupied and declared to be a "liberated" city. On 22 September, the Syrians withdrew from Irbid after suffering heavy losses to a coordinated aerial–ground offensive by the Jordanians. Mounting pressure from other Arab countries, such as Iraq, led Hussein to halt his offensive. On 13 October, he signed an agreement with Arafat to regulate the fedayeen's presence in Jordan. However, the Jordanian military attacked again in January 1971, and the fedayeen were driven out of the cities, until 2,000 surrendered after they were encircled during the Ajlun offensive on 23 July, formally marking the end of the conflict.

Jordan allowed the fedayeen to relocate to Lebanon via Syria, where they later became involved in the Lebanese Civil War. The Palestinian Black September Organization was founded after the conflict to carry out attacks against Jordanian authorities in response to the fedayeen's expulsion; their most notable attack was the assassination of Jordanian prime minister Wasfi Tal in 1971, as he had commanded parts of the military operations against the fedayeen. The following year, the organization shifted its focus to attacking Israeli targets and carried out the Munich massacre against Israeli athletes. Though the events of Black September did not reflect a Jordanian–Palestinian divide, as there were Jordanians and Palestinians on both sides of the conflict, it paved the way for such a divide to emerge after the conflict.

After the Jordanian annexation of the West Bank in 1950, it conferred its citizenship on the West Bank Palestinians. The combined population of the West Bank and Jordan consisted of two-thirds Palestinians (one-third in the West Bank and one-third in the East Bank) and one-third Jordanians. Jordan provided Palestinians with seats amounting to half the parliament, and Palestinians enjoyed equal opportunities in all sectors of the state. This demographic change influenced Jordanian politics.

King Hussein considered that the Palestinian problem would remain the country's overriding national security issue; he feared an independent West Bank under PLO administration would threaten the autonomy of his Hashemite kingdom. The Palestinian factions were supported vicariously by many Arab governments, most notably Egypt's President Gamal Abdel Nasser, who gave them political support.

The Palestinian nationalist organization Fatah started organizing cross-border attacks against Israel in January 1965, often drawing severe Israeli reprisals upon Jordan. The Samu incident launched by Israel on 13 November 1966 was one such reprisal, taking place after three Israeli soldiers were killed by a Fatah landmine. The Israeli assault on the Jordanian-controlled West Bank town of As-Samu inflicted heavy casualties on Jordan. Israeli writer Avi Shlaim argued that Israel's disproportionate retaliation exacted revenge on the wrong party, as Israeli leaders knew from their interaction with Hussein that he was doing everything he could to prevent such attacks. Hussein, who felt he had been betrayed by the Israelis, drew fierce local criticism because of this incident. It is thought that this contributed to his decision to join Egypt and Syria's war against Israel in 1967. In June 1967 Israel captured the West Bank from Jordan during the Six-Day War.

After Jordan lost the West Bank, Fatah (under the PLO) stepped up their guerrilla attacks against Israel from Jordanian soil, making the border town of Karameh their headquarters. On 18 March 1968, an Israeli school bus was blown up by a mine near Be'er Ora in the Arava, killing two adults and wounding ten children—the 38th Fatah operation in little more than three months. On 21 March, Israel Defense Forces (IDF) units entered Jordan and launched a reprisal attack on Karameh that developed into a full-scale battle that lasted a day. The PLO suffered some 200 casualties and another 150 taken prisoner; 40–84 Jordanian soldiers were also killed. Israeli losses stood at around 30 killed and 69–161 wounded, and they also left behind several vehicles.