The Deir Yassin massacre took place on April 9, 1948, when around 130 fighters from the Zionist paramilitary groups Irgun and Lehi killed at least 107 Palestinian Arabs, including women and children, in Deir Yassin, a village of roughly 600 people near Jerusalem. The assault occurred as Jewish militia sought to relieve the blockade of Jerusalem during the civil war that preceded the end of British rule in Palestine.

The villagers put up stiffer resistance than the Jewish militias had expected and they suffered casualties. The village fell after house-to-house fighting. Some of the Palestinian Arabs were killed in the course of the battle, others while trying to flee or surrender. A number of prisoners were executed, some after being paraded in West Jerusalem. In addition to the killing and widespread looting, there may have been cases of mutilation and rape. Despite an original boast by the victors that 254 had been killed, modern scholarship puts the death toll at far fewer. Palestinian historian Aref al-Aref counted 117 victims, seven in combat and the rest in their homes. The number of wounded is estimated to between 12 and 50. Five of the attackers were killed and a dozen wounded.The massacre was condemned by the leadership of the Haganahthe Jewish community's main paramilitary forceby the area's two chief rabbis and famous Jews abroad like Albert Einstein, Jessurun Cardozo, Hannah Arendt, Sidney Hook and others. The Jewish Agency for Israel sent Jordan's King Abdullah a letter of apology, which he rebuffed. He held them responsible for the massacre, and warned about "terrible consequences" if similar incidents occurred elsewhere.The massacre became a pivotal event in the ArabIsraeli conflict for its demographic and military consequences. The narrative was embellished and used by various parties to attack each otherby the Palestinians against Israel; by the Haganah to play down their own role in the affair; and by the Israeli left to accuse the Irgun and Lehi of blackening Israel's name by violating the Jewish principle of purity of arms. News of the killings sparked terror among Palestinians, frightening them to flee their homes in the face of Jewish troop advances and it strengthened the resolve of Arab governments to intervene, which they did five weeks later. Four days after the Deir Yassin massacre, on April 13, a revenge attack on the Hadassah medical convoy in Jerusalem ended in a massacre killing 78 Jews, most of whom were the medical staff. Archival material in Israeli military deposits documenting the massacre remain classified.

The Irgun (Hebrew: ארגון; full title: Hebrew: הארגון הצבאי הלאומי בארץ ישראל Hā-ʾIrgun Ha-Tzvaʾī Ha-Leūmī b-Ērētz Yiśrāʾel, lit. "The National Military Organization in the Land of Israel") was a Zionist paramilitary organization that operated in Mandate Palestine between 1931 and 1948. The organization is also referred to as Etzel (Hebrew: אצ"ל), an acronym of the Hebrew initials, or by the abbreviation IZL. It was an offshoot of the older and larger Jewish paramilitary organization Haganah (Hebrew: Hebrew: הגנה, Defence). When the group broke from the Haganah it became known as the Haganah Bet (Hebrew: literally "Defense 'B' " or "Second Defense", Hebrew: הגנה ב), or alternatively as haHaganah haLeumit (Hebrew: ההגנה הלאומית) or Hama'amad (Hebrew: המעמד). Irgun members were absorbed into the Israel Defense Forces at the start of the 1948 Arab–Israeli war.

The Irgun policy was based on what was then called Revisionist Zionism founded by Ze'ev Jabotinsky. According to Howard Sachar, "The policy of the new organization was based squarely on Jabotinsky's teachings: every Jew had the right to enter Palestine; only active retaliation would deter the Arabs; only Jewish armed force would ensure the Jewish state".Two of the operations for which the Irgun is best known are the bombing of the King David Hotel in Jerusalem on 22 July 1946 and the Deir Yassin massacre, carried out together with Lehi on 9 April 1948.

The Irgun has been viewed as a terrorist organization or organization which carried out terrorist acts. Specifically the organization "committed acts of terrorism and assassination against the British, whom it regarded as illegal occupiers, and it was also violently anti-Arab" according to the Encyclopædia Britannica. In particular the Irgun was described as a terrorist organization by the United Nations, British, and United States governments; in media such as The New York Times newspaper; as well as by the Anglo-American Committee of Inquiry, the 1946 Zionist Congress and the Jewish Agency. However, academics such as Bruce Hoffman and Max Abrahms have written that the Irgun went to considerable lengths to avoid harming civilians, such as issuing pre-attack warnings; according to Hoffman, Irgun leadership urged "targeting the physical manifestations of British rule while avoiding the deliberate infliction of bloodshed." Albert Einstein, in a letter to the New York Times in 1948, compared Irgun and its successor Herut party to "Nazi and Fascist parties" and described it as a "terrorist, right wing, chauvinist organization". Irgun's tactics appealed to many Jews who believed that any action taken in the cause of the creation of a Jewish state was justified, including terrorism.The Irgun was a political predecessor to Israel's right-wing Herut (or "Freedom") party, which led to today's Likud party. Likud has led or been part of most Israeli governments since 1977.