Founding
The Irgun's first steps were in the aftermath of the Riots of 1929.
https://en.wikipedia.org/wiki/Irgun#Revolt
The riots took the form, for the most part, of attacks by Arabs on Jews accompanied by destruction of Jewish property. During the week of riots, from 23 to 29 August, 133 Jews were killed by Arabs, and 339 Jews were injured, most of whom were unarmed.[1][2] There were 116 Arabs killed and at least 232 wounded, mostly by the Mandate police suppressing the riots.
https://en.wikipedia.org/wiki/1929_Palestine_riots