The 840s was the decade from 1 January 840 to 31 December 849. It was a period of political fragmentation and shifting power across Eurasia. In Western Europe the death of an emperor and the civil war among his heirs set the stage for new territorial boundaries. In the north Atlantic and along European coasts Norse seafarers increased raids and began to establish permanent bases. Farther east, religious and court politics in Byzantium and Tang China brought important cultural changes. The Abbasid Caliphate also experienced succession and policy shifts that affected the Islamic world.

Major events and turning points

  • Death of Emperor Louis the Pious (840) and the ensuing struggle of his sons, culminating in the Treaty of Verdun (843), which partitioned the Carolingian Empire among Lothair I, Charles the Bald and Louis the German.
  • Viking incursions intensified: the long-range raid on Paris (845) and the establishment of Norse trading and settlement centers such as Dublin (commonly dated to 841) marked a transition from seasonal raiding to colonization and commerce.
  • Byzantine restoration of icon veneration (often called the Triumph of Orthodoxy) in 843 under the regency of Empress Theodora, ending the period of official iconoclasm that had divided the empire.
  • Tang China under Emperor Wuzong pursued a harsh anti-Buddhist campaign in the early-to-mid 840s that led to the suppression of many monasteries and redistribution of wealth and land.
  • Changes in the Abbasid caliphate: the death of al-Mu'tasim (842) and the succession of al-Wathiq, followed later by al-Mutawakkil (beginning 847), brought new court dynamics and shifting policies at the heart of the caliphate.

Politically, the decade accelerated the decentralization of power in Western Europe. The Treaty of Verdun is often seen as a formative moment in the long-term emergence of distinct western and eastern Frankish polities that would evolve into medieval France and Germany. Local magnates and regional rulers gained greater autonomy as central Carolingian authority weakened.

Economically and socially, the 840s reflected growing connectivity and stress. Viking activity opened new trade routes even as raids disrupted coastal and riverine communities. In Asia, state campaigns such as the Tang anti-Buddhist measures altered landholding patterns and religious institutions, while in the Islamic world administrative and fiscal adjustments followed caliphal changes.

Notable figures of the decade include Louis the Pious and his sons Lothair I, Charles the Bald and Louis the German; Byzantine regent Theodora; Tang emperor Wuzong; and Abbasid caliphs al-Mu'tasim, al-Wathiq and al-Mutawakkil. Their actions—on battlefields, in courts, and through decrees—shaped developments that resonated beyond the decade.

Legacy: the 840s left several durable marks on medieval history. The partition of the Carolingian realm helped define territorial identities in Europe; Viking settlements developed into political and economic centers; the Byzantine decision to restore icon veneration oriented eastern Christendom; and the Tang and Abbasid policies each influenced religious and administrative life in Asia. These converging changes make the 840s a crossroads decade in the transformation from early to high medieval world structures.