List of state leaders in 840
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839 state leaders - Events of 840 - 841 state leaders - State leaders by year

Asia

• Bohai/Balhae - Da Yizhen/Dae I-jin, King of Bohai/Balhae (830-857)
• China (Tang Dynasty) -
  1. Wenzong, Emperor of Tang China (826-840)
  2. Wuzong, Emperor of Tang China (840-846)
• Gurjara - Mihira Bhoja I, Pratihara King of Gurjara (c. 836-886)
• Japan (Heian Period) - Emperor Nimmyō, emperor of Japan (833-850)
• Silla - Munseong, King of Silla (839-857)

Europe

• Al-Andalus - Abd-ar-rahman II, Emir of Cordoba (822-852)
• Asturias - Alfonso II, King of Asturias (791-842)
• Bavaria - Louis the German, King of Bavaria (817-843)
• Benevento - Ralchedis I, Prince of Benevento (839-851)
• Bulgaria - Presian, Khan of Bulgaria (836-852)
• Dál Riata - Alpin II, King of Dalriada (839–842)
• East Anglia¹ - Æthelweard, King of East Anglia (839–855)
• England - Æthelwulf of Wessex, Bretwalda (839–856)
• Frankish Empire -
  • Louis the Pious, Holy Roman Emperor (816-840)
  • Lothair I, Holy Roman Emperor (840-855)
• Mercia -
  1. Wiglaf, King of Mercia (830–840)
  2. Wigstan, King of Mercia (840–840)
  3. Beorhtwulf, King of Mercia (840–852)
• Middle Francia - Lothair I, King of Middle Francia (840-855), Holy Roman Emperor (823-855)
• Northumbria¹ - Eanred, King of Northumbria (810–841)
• Ireland -
  • Nial Caille, High King of Ireland (832–844)
  • Turgesius of Norway, High King of Ireland (832–845)
• Papal States - Gregory IV, Pope (827-844)
• Picts - Uurad, King of the Picts (839–842)
• Strathclyde - Arthgal f Dyfnwal, King of Strathclyde (816–872)
• Venice - Pietro Tradonico, Doge of Venice (837-864)
• Wales - Merfyn Frych ap Gwriad, King of Gwynedd (825–844)
• West Francia - Charles the Bald, King of West Francia (840-877)

Middle East

• Abbasid Caliphate - Al-Mu'tasim, Caliph (833-842)
• Byzantine Empire - Theophilus, Byzantine Emperor (829-842)

Notes

1. Though East Anglia, Northumbria and Mercia are traditionally perceived to have been in the heptarchy they were largely independent kingdoms until its conquest by the Danes in 869 and 867, respectively - where Mercia was conquered by Wessex in 874. All other members of the heptarchy had been conquered by Wessex and by this time had no form or king.