Black September (6 September 1970-June 1971) was a conflict fought between the guerrillas of the Palestine Liberation Organization and the Jordanian government's armed forces from September 1970 to June 1971. The war, which was fought as a sort of civil war, saw the Jordanian government force the troublesome PLO fedayeen to end their occupation of much of Jordan and to retreat into southern Lebanon, where they moved their bases. Black September was seen as a pivotal moment in Palestinian history, as PLO leader Yasser Arafat decided to ally with the Arab monarchies against Israel instead of attempting to launch leftist revolutions against them.
In the aftermath of the Arab-Israeli War, Jordan occupied the West Bank in the Palestine region, and Palestinians would come to make up a vast majority (70%) of the population of Jordan. In the aftermath of the 1967 Six-Day War, the Palestine Liberation Organization moved its bases into Jordan, fleeing the Israeli-occupied West Bank and entrenching itself in the large refugee camps in the Jordanian desert and near the capital of Amman. King Hussein I of Jordan feared the might of the PLO, which was seen as a "state within a state" due to its strength and its refusal to assimilate. The PLO fought valiantly against an Israeli cross-border raid at Karameh, but the Jordanian government was concerned about the PLO presence in the "occupied territories" in the desert. Thousands flocked to join the Palestinian guerrilla groups in Jordan, and many leaders of the PLO considered overthrowing King Hussein and putting the Palestinian resistance movement in power.
In 1970, the PLO decided to call for a general strike to pressure the government into creating a People's Assembly that included Palestinian representatives, but King Hussein refused to give in to the demands of the occupying Palestinian guerrillas. Arafat declared that the "Palestinian revolution" would fight to defend itself until the "fascist military government" was overthrown, but the Jordanian army was sent in to crush the PLO. The leaders of Fatah believed that armies from neighboring Syria and Ba'athist Iraq would assist in the revolution against the Jordanian government, but a half-hearted attempt by the Syrian Arab Army to move tanks into Jordan was repelled, and the 25,000 Iraqi troops stationed in Jordan remained passive as the Jordanian government crushed the PLO. On 27 September 1970, Arafat and King Hussein agreed to end the fighting after Egyptian leader Gamal Abdel Nasser convinced them to sit down, but Nasser's death allowed for Hussein to launch a counterattack against the PLO, driving them from Amman and other cities. Jordanian tanks, aircraft, and heavy artillery pushed the Palestinians to the north of the country, and 3,000 Palestinians died during the final assault on the Palestinian strongholds of Jerash and Ajlun in July 1971. Black September led to the PLO relocating to South Lebanon, setting up refugee camps and military bases for cross-border raids into Israel. Vengeful Palestinian militants would create the "Black September Organization" with the goal of fighting back against the Jordanian government and the "imperialist" forces responsible for the PLO's defeat, assassinating Prime Minister Wasfi al-Tal for his role in torturing and killing Abu Ali Iyad and carrying out terrorist attacks against Israeli and Western targets abroad. The war also led to a revenge attack against the Jordanian embassy in Tehran, Iran by the People's Mujahideen of Iran, which was anti-Zionist and opposed the Jordanian government's war on the PLO.