Complex sentence: Henry and Eleanor mobilized their own supporters and raised a foreign mercenary army; facing the threat of open civil war, the barons backed down: de Clare switched sides once again, de Montfort left for exile in France and the baronial resistance collapsed.
Split into simpler sentences: Henry and Eleanor mobilized their own supporters and raised a foreign mercenary army. Facing the threat of open civil war, the barons backed down: de Clare switched sides once again, de Montfort left for exile in France and the baronial resistance collapsed.