A crusader victory over pagan Estonian forces on 15 June 1219 fought near Reval (mod. Tallinn, Estonia), which secured a stronghold for the Danish subjugation of northern Estonia in the years following.
In order to fulfill a crusade vow, and possibly in response to pleas for assistance from Bishop Albert of Riga, King Valdemar II of Denmark sailed to Reval with an army of Danish, German, and Wendish forces comprising some 100 ships and 3,000 men with the intention of subjecting northern Estonia to Danish rule. Accompanying the army were Anders Sunesen, archbishop of Lund; Peter Sunesen, bishop of Roskilde; Niels, bishop of Schleswig; and Theoderic, the nominal bishop of Estonia.
Soon after their arrival the crusaders tore down the wooden fortress of Lyndanise outside Reval and built a stone castle on the site. After three days of negotiations, Estonian forces attacked the Christian camp and killed, among others, Bishop Theoderic, who had been mistaken for the Danish king. The crusaders were ultimately saved only by a late effort from the Wendish forces in the army, which were under the subcommand of Wizlaw, prince of Rügen.
According to intellectual constructions dating from the sixteenth century, which entered Danish popular belief, a flag bearing a white cross on a red ground is supposed to have fallen miraculously from the sky at a critical point in the battle; this was held to be the origin of the Dannebrog, the Danish national flag.