Otto IV (1175 19 May 1218) was the Holy Roman Emperor from 1209 until his death in 1218.

Otto spent most of his early life in England and France. He was a follower of his uncle Richard the Lionheart, who made him Count of Poitou in 1196. With Richard's support, he was elected King of Germany by one faction in a disputed election in 1198, sparking ten years of civil war. The death of his rival, Philip of Swabia, in 1208 left him sole king of Germany.

In 1209, Otto marched to Italy to be crowned emperor by Pope Innocent III. In 1210, he sought to unite the Kingdom of Sicily with the Empire, breaking with Innocent, who excommunicated him. He allied with England against France and took part in the alliance's defeat at Bouvines in 1214. He was abandoned by most of his supporters in 1215 and lived the rest of his life in retirement on his estates near Brunswick. He was the only German king of the Welf dynasty.

Pope Innocent III (Latin: Innocentius III; 1160 or 1161 – 16 July 1216), born Lotario dei Conti di Segni (anglicized as Lothar of Segni), was the head of the Catholic Church and ruler of the Papal States from 8 January 1198 to his death in 16 July 1216.

Pope Innocent was one of the most powerful and influential of the medieval popes. He exerted a wide influence over the Christian states of Europe, claiming supremacy over all of Europe's kings. He was central in supporting the Catholic Church's reforms of ecclesiastical affairs through his decretals and the Fourth Lateran Council. This resulted in a considerable refinement of Western canon law. He is furthermore notable for using interdict and other censures to compel princes to obey his decisions, although these measures were not uniformly successful.

Innocent greatly extended the scope of the Crusades, directing crusades against Muslim Iberia and the Holy Land as well as the Albigensian Crusade against the Cathars in southern France.

He organized the Fourth Crusade of 1202–1204, which ended in the sack of Constantinople. Although the attack on Constantinople went against his explicit orders, and the Crusaders were subsequently excommunicated, Innocent reluctantly accepted this result, seeing it as the will of God to reunite the Latin and Eastern Orthodox Churches. In the event, the sack of Constantinople and the subsequent period of Frankokratia heightened the hostility between the Latin and Greek churches. (The Byzantine Empire was restored in 1261 but never regained its former strength, finally falling in 1453.)