 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 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 ) . 
