In response to numerous Arab attacks on Jewish communities, the Haganah, a Jewish paramilitary organisation, was formed on 15 June 1920 to defend Jewish residents. Tensions led to widespread violent disturbances on several occasions, notably in 1921 (see Jaffa riots), 1929 (primarily violent attacks by Arabs on Jewssee 1929 Hebron massacre) and 1936–1939. Beginning in 1936, Jewish groups such as Etzel (Irgun) and Lehi (Stern Gang) conducted campaigns of violence against British military and Arab targets.