Complex sentence: William of Rubruck's was the fourth European mission to the Mongols: previous ones were led by Giovanni da Pian del Carpine and Ascelin of Lombardia in 1245 and André de Longjumeau in 1249.
Split into simpler sentences: William of Rubruck's was the fourth European mission to the Mongols. Before William of Rubruck went Giovanni da Pian del Carpine and Ascelin of Lombardia in 1245 and André de Longjumeau in 1249.