 He gained fame as a guerrilla warfare strategist, planning and directing many successful attacks on British forces, such as the "Bloody Sunday" assassinations of key British intelligence agents in November 1920.After the July 1921 ceasefire, Collins and Arthur Griffith were sent to London by Éamon de Valera to negotiate peace terms.The resulting Anglo-Irish Treaty established the Irish Free State but depended on an Oath of Allegiance to the Crown, a condition with which de Valera and other republican leaders had difficulty reconciling.