From 1186 to 1258 Bulgaria experienced temporary resuscitation, the brevity of which was more than compensated for by the stirring nature of the events that crowded it. The exactions and oppressions of the Greeks culminated in a revolt on the part of the Bulgars, which had its centre in Tirnovo on the river Yantra in northern Bulgaria—a position of great natural strength and strategic importance, commanding the outlets of several of the most important passes over the Balkan range. This revolt coincided with the growing weakness of the eastern empire, which, surrounded on all sides by aggressive enemies—Kumans, Saracens, Turks, and Normans—was sickening for one of the severe illnesses which preceded its dissolution. The revolt was headed by two brothers who were Vlakh or Rumanian shepherds, and was blessed by the archbishop Basil, who crowned one of them, called John Asen, as _tsar_ in Tirnovo in 1186. Their first efforts against the Greeks were not successful, but securing the support of the Serbs under Stephen Nemanja in 1188 and of the Crusaders in 1189 they became more so; but there was life in the Greeks yet, and victory alternated with defeat. John Asen I was assassinated in 1196 and was succeeded after many internal discords and murders by his relative Kaloian or Pretty John. This cruel and unscrupulous though determined ruler soon made an end of all his enemies at home, and in eight years achieved such success abroad that Bulgaria almost regained its former proportions. Moreover, he re-established relations with Rome, to the great discomfiture of the Greeks, and after some negotiations Pope Innocent III recognized Kaloian as _tsar_ of the Bulgars and Vlakhs (roi de Blaquie et de Bougrie, in the words of Villehardouin), with Basil as primate, and they were both duly consecrated and crowned by the papal legate at Tirnovo in 1204. The French, who had just established themselves in Constantinople during the fourth crusade, imprudently made an enemy of Kaloian instead of a friend, and with the aid of the Tartar Kumans he defeated them several times, capturing and brutally murdering Baldwin I. But in 1207 his career was cut short; he was murdered while besieging Salonika by one of his generals who was a friend of his wife. After eleven years of further anarchy he was succeeded by John Asen II. During the reign of this monarch, which lasted from 1218 till 1241, Bulgaria reached the zenith of its power. He was the most enlightened ruler the country had had, and he not only waged war successfully abroad but also put an end to the internal confusion, restored the possibility of carrying on agriculture and commerce, and encouraged the foundation of numerous schools and monasteries. He maintained the tradition of his family by making his capital at Tirnovo, which city he considerably embellished and enlarged.