September 1970 is known as the Black September in Arab history and sometimes is referred to as the "era of regrettable events." It was a month when Hashemite King Hussein of Jordan moved to quash an attempt by Palestinian organizations to overthrow his monarchy. The violence resulted in heavy civilian Palestinian casualties. Armed conflict lasted until July 1971 with the expulsion of the PLO and thousands of Palestinians to Lebanon.

On September 16, King Hussein declared martial law. The next day, Jordanian tanks (the 60th armored brigade) attacked the headquarters of Palestinian organizations in Amman; the army also attacked camps in Irbid, Salt, Sweileh and Zarqa. Then the head of Pakistani training mission to Jordan, Brigadier Muhammad Zia-ul-Haq (later Chief of Army Staff and President of Pakistan), took command of the 2nd division. The armored troops were inefficient in narrow city streets and thus the Jordanian army conducted house to house sweeps for Palestinian fighters and got immersed in heavy urban warfare with the inexperienced and undisciplined Palestinian fighters.

On September 18, Syria, through the Palestine Liberation Army (PLA) branch, whose headquarters were located in Damascus and which was very close to the Syrian regime, tried to intervene on the behalf of the Palestinian guerrillas. The PLA size was equivalent to a division and was met by the 40th armored brigade of the Jordanian army. The Jordanian king asked for American support to prevent the Syrian-backed attack which could ultimately result in a victory of the Palestinians and an end to his pro-western government. In order to protect their vital Arab ally, the American government requested Israeli help. Israel Air Force planes made low overflights over the PLA tanks as a sign of warning. Soon the PLA began to withdraw.

Meanwhile, both Hussein and Arafat attended the meeting of leaders of Arab countries in Cairo and on September 27 Hussein signed an agreement that treated both sides as equals and acknowledged the right of the Palestinian organizations to operate in Jordan, but they should leave the cities and stay in the fronts. On September 28, Egypt's Nasser died of a sudden heart attack. Thus, the PLO became without Arabic protection, and King Hussein continued the attack.

On October 31, Arafat, whose position was weakened, had to sign another agreement (similar to one of November 1968) that returned control of Jordan to the King, requiring the dismantlement of Palestinian militant bases and banning their members from carrying unconcealed weapons. At a meeting of the Palestinian National Council that followed, both PFLP and DFLP groups refused to accept this agreement and instead, accepted the proposal that Jordan would be a part of a Palestinian state to replace both Jordan and Israel.

The violations continued and on November 9, Jordanian prime minister Wasfi al-Tal signed an order to confiscate illegal weapons. By January 1971, the army strengthened its control over the cities. Another agreement regarding surrendering weapons was signed and broken. After the discovery of illegal arms warehouse in Irbid in the Spring, the army placed a curfew and began arresting the rebels. On June 5, several leading Palestinian organizations including Arafat's Fatah, called on Radio Baghdad to overthrow King Hussein who was regarded as a "puppet separatist authority."

The army regained control over the remaining PLO strongholds, mountainous cities of Jerash and Ajloun. Palestinian militants were driven out to Lebanon as a result of the Cairo Agreement.