Black September (Arabic: أيلول الأسود; Aylūl Al-Aswad) was the conflict fought in Jordan between the Jordanian Armed Forces (JAF), under the leadership of King Hussein, and the Palestine Liberation Organisation (PLO), under the leadership of Yasser Arafat, primarily between 16 and 27 September 1970, with certain actions continuing until 17 July 1971.
After Jordan lost control of the West Bank to Israel in 1967, Palestinian fighters known as fedayeen moved their bases to Jordan and stepped up their attacks on Israel and Israeli-occupied territories. One Israeli retaliation on a PLO camp based in Karameh, a Jordanian town along the border with the West Bank, developed into a full-scale battle. The perceived joint Jordanian-Palestinian victory in the 1968 Battle of Karameh led to an upsurge in Arab support for the Palestinian fighters in Jordan. The PLO's strength in Jordan grew, and by the beginning of 1970, groups within the PLO had started to openly call for the overthrow of the Hashemite monarchy. Acting as a state within a state, the fedayeen disregarded local laws and regulations, and even attempted to assassinate King Hussein twice—leading to violent confrontations between them and the Jordanian army in June 1970. Hussein wanted to oust the fedayeen from the country, but hesitated to strike because he did not want his enemies to use it against him by equating Palestinian fighters with civilians. PLO actions in Jordan culminated in the Dawson's Field hijackings incident of 10 September, in which the fedayeen hijacked three civilian aircraft and forced their landing in Zarqa, taking foreign nationals as hostages, and later blowing up the planes in front of international press. Hussein saw this as the last straw, and ordered the army to move.
On 17 September, the Jordanian army surrounded cities with a PLO presence including Amman and Irbid, and began shelling the fedayeen, who had established themselves in Palestinian refugee camps. The next day, a Syrian force with Palestine Liberation Army markings, intervened in support of the fedayeen. It advanced towards Irbid to which the fedayeen had declared a "liberated" city. On 22 September, the Syrians withdrew from Irbid after the Jordanian army launched an air-ground offensive that inflicted heavy Syrian losses. Pressure mounted by Arab countries led Hussein to halt the fighting. On 13 October he signed an agreement with Arafat to regulate the fedayeen's presence. However, the Jordanian army attacked again in January 1971. The fedayeen were driven out of the cities, one by one, until 2,000 fedayeen surrendered after being surrounded in a forest near Ajloun on 17 July, marking the end of the conflict.
Jordan allowed the fedayeen to leave for Lebanon via Syria, and the fedayeen later participated in the 1975 Lebanese Civil War. The Black September Organization was founded after the conflict to carry out reprisals against the Jordanian authorities. The organization's first attack was the assassination of Wasfi Al-Tal in 1971, the then Jordanian Prime Minister who had commanded parts of the operation that expelled the fedayeen. The organization then shifted to attacking Israeli targets, including the highly publicized 1972 Munich massacre of Israeli athletes.