John of Brienne, also known as John I, was King of Jerusalem from 1210 to 1225 and Latin Emperor of Constantinople from 1229 to 1237. He was the youngest son of Erard II of Brienne, a wealthy nobleman in Champagne. John, originally destined for an ecclesiastical career, became a knight and owned small estates in Champagne around 1200. After the death of his brother, Walter III, he ruled the County of Brienne on behalf of his minor nephew Walter IV (who lived in southern Italy).
The barons of the Kingdom of Jerusalem proposed that John marry Maria, Queen of Jerusalem. With the consent of Philip II of France and Pope Innocent III, he left France for the Holy Land and married the queen; the royal couple were crowned in 1210. After Maria's death in 1212 John administered the kingdom as regent for their infant daughter, Isabella II; an influential lord, John of Ibelin, attempted to dethrone him. John was a leader of the Fifth Crusade. Although his claim of supreme command of the crusader army was never unanimously acknowledged, his right to rule Damietta (in Egypt) was confirmed shortly after the town fell to the crusaders in 1219. He claimed the Armenian Kingdom of Cilicia on behalf of his second wife, Stephanie of Armenia, in 1220. After Stephanie and their infant son died that year, John returned to Egypt. The Fifth Crusade ended in failure (including the recovery of Damietta by the Egyptians) in 1221.
John was the first king of Jerusalem to visit Europe (Italy, France, England, León, Castile and Germany) to seek assistance for the Holy Land. He gave his daughter in marriage to Holy Roman Emperor Frederick II in 1225, and Frederick ended John's rule of the Kingdom of Jerusalem. Although the popes tried to persuade Frederick to restore the kingdom to John, the Jerusalemite barons regarded Frederick as their lawful ruler. John administered papal domains in Tuscany, became the podestà of Perugia and was a commander of Pope Gregory IX's army during Gregory's war against Frederick in 1228 and 1229.
He was elected emperor in 1229 as the senior co-ruler (with Baldwin II) of the Latin Empire, and was crowned in Constantinople in 1231. John III Vatatzes, Emperor of Nicaea, and Ivan Asen II of Bulgaria occupied the last Latin territories in Thrace and Asia Minor, besieging Constantinople in early 1235. John directed the defence of his capital during the months-long siege, with the besiegers withdrawing only after Geoffrey II of Achaea and united fleets from Italian towns defeated their fleet in 1236. The following year, John died as a Franciscan friar.