11 events the sources record for this year, grouped by region.
What happened
China (3)
Wang Chengzong rebelled, and the eunuch Tutu Chengcui was made Commissioner for Field Command of Zhenzhou to lead the suppression. (Old Book of Tang (Jiu Tangshu), Annals 舊唐書 卷14 元和4年)
Wang Chengzong, military governor of the Chengde Army, rebelled; Tutu Chengcui was appointed pacification commissioner to suppress him. (New Book of Tang (Xin Tangshu), Annals 新唐書 卷007 德宗·順宗·憲宗 元和4年10月)
Tutu Chengcui leaves Chang'an with the Shence army and orders the surrounding regional military governments to join the campaign. (Zizhi Tongjian (Comprehensive Mirror) 資治通鑑 卷238)
Japan (4)
Emperor Heizei, unable to attend to government because of wind disease (fūbyō), issued an edict abdicating in favor of his younger brother, the Crown Prince. (Nihon Kōki (Later Chronicles of Japan) 日本後紀 卷17)
The retired emperor traveled by river in a paired boat to Heijō; since the palace was not yet finished, he temporarily stayed at the house of the late Minister of the Right Ōnakatomi no Kiyomaro. (Nihon Kōki (Later Chronicles of Japan) 日本後紀 卷18)
Two thousand five hundred craftsmen and laborers were hired from the Home Provinces (Kinai) and other provinces to build the Heijō palace. (Nihon Kōki (Later Chronicles of Japan) 日本後紀 卷18)
West Asia (1)
Al-Amin takes Harun's decree from the Kaaba and tears it up (Al-Tabari, History of the Prophets and Kings Series III tom.2 (Arg.) [india.history.resource.86977], Annus 194 (OCR 「104」))
Southern Europe (1)
In the west King Ludovicus enters Spain with an army and besieges Tortosa on the Ebro River; as it does not fall easily, he lifts the siege and returns safely to Aquitaine (Royal Frankish Annals ARF a.809, Kurze 1895 p.127)
Western Europe (2)
In November the emperor holds a council at Aachen on the question of the procession of the Holy Spirit, and sends Bernarius, bishop of Worms, and Adalhard, abbot of Corbie, to Pope Leo (Royal Frankish Annals ARF a.809, Kurze 1895 p.129)
Just as he is about to build a city across the Elbe in response to the Danish king's arrogance, Trasco, Duke of the Abodrites, is murdered at the trading place of Reric by Godofrid's men. Count Egbert occupies the site of Esesfeld on the bank of the Stura River around March 15 and begins to build (Royal Frankish Annals ARF a.809, Kurze 1895 p.129)