After arriving in India in 1915, Mahatma Gandhi
successfully organised Satyagraha movements in various places. These places included Champaran in Bihar (1916), Kheda district of Gujarat (1917) and Ahmedabad in Gujarat (1918). Emboldened with this success, Gandhiji in 1919 decided to launch a nationwide Satyagraha against the proposed Rowlatt Act (1919).
According to this law, the government had the authority to repress political activities, and allowed detention of political prisoners-without trial for two years.
Gandhiji wanted a non-violent civil disobedience i.e. Satyagraha movement against such unjust law all over India. Thus, it started with a hartal on 6th April, 1919.