Rumi calendar in the context of "31 March incident"

The 31 March incident (Turkish: 31 Mart Vakası) was an uprising in the Ottoman Empire in April 1909, during the Second Constitutional Era. The incident broke out during the night of 30–31 Mart 1325 in Rumi calendar (GC 12–13 April 1909), thus named after 31 March where March is the equivalent to Rumi month Mart. Occurring soon after the 1908 Young Turk Revolution, in which the Committee of Union and Progress (CUP) had successfully restored the Constitution and ended the absolute rule of Sultan Abdul Hamid II (r. 1876–1909), it is sometimes referred to as an attempted countercoup or counterrevolution. It consisted of a general uprising against the CUP within Istanbul, largely led by reactionary groups, particularly Islamists opposed to the secularising influence of the CUP and supporters of absolutism, although liberal opponents of the CUP within the Liberty Party also played a lesser role. Eleven days later the uprising was suppressed and the former government restored when elements of the Ottoman Army sympathetic to the CUP formed an impromptu military force known as the Action Army (Hareket Ordusu). Upon entering Istanbul on 24 April Sultan Abdul Hamid II, accused by the CUP of complicity in the uprising, was deposed and the Ottoman National Assembly elevated his half-brother, Mehmed V, to the throne. Mahmud Shevket Pasha, the military general who had organised and led the Action Army, became the most influential figure in the restored constitutional system until his assassination in 1913.

The precise nature of events is uncertain; differing interpretations have been offered by historians, ranging from a spontaneous revolt of discontents to a secretly planned and coordinated counter-revolution against the CUP. Most modern studies disregard claims the sultan was actively involved in plotting the uprising, emphasising the CUP's mismanagement of troops in the build up to the mutiny and the role of conservative religious groups. The crisis was an important early moment in the empire's process of disintegration, setting a pattern of political instability which continued with military coups in 1912 and 1913. The temporary loss of power led to radicalisation within the CUP, resulting in an increasing willingness among Unionists to utilise violence. Some scholars have argued that the deterioration of ethnic relations and erosion of public institutions during 1908–1909 precipitated the Armenian genocide. The crisis also represented the demise of the Sultanate's power in the Ottoman Empire, as a series of constitutional amendments confined its function in government to the confirmation of parliamentary decisions, conversely cementing parliament's supremacy in a significant step of republicanism in Turkish political history.