 John of Brienne ( c . 1170 – 27 March 1237 ) , 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 crusader in 1190 and joined the Third Crusade in 1191 . He participated in the siege of Acre and was taken prisoner by Saladin in 1192 . After his release in 1194 , he participated in the Crusade