In the early 20th century Michael Collins and Tom Barry both developed many tactical features of guerrilla warfare during the guerrilla phase of the 1919-1921 Irish War of Independence. Collins developed mainly urban guerrilla-warfare tactics in Dublin city (the Irish capital). Operations in which small Irish Republican Army (IRA) units (3 to 6 guerrillas) quickly attacked a target and then melted back into civilian crowds frustrated the British enemy, who often killed Irish civilians in revenge for attacks on British Crown Forces - which just made more Irish people support the Irish Republican Army guerrillas. The best example of this occurred on Bloody Sunday (21 November 1920), when Collins's assassination unit, known as "The Squad", wiped out a group of British intelligence agents ("the Cairo Gang") early in the morning (14 were killed, six were wounded) - some regular officers were also killed in the purge. That afternoon a British force of mixed units took revenge ,shooting at a crowd at a football match in Croke Park, killing fourteen civilians and injuring 60 others.