Complex sentence: He achieved the serment that he made with his soldiers, after the decisive Battle of Kufra were he made the Oath of Kuffra, were he swore that they will keep up the fight until the french flag will flies over the Cathedral of Strasbourg.
Split into simpler sentences: He achieved the serment that he made with his soldiers, after the decisive Battle of Kufra. With the Oath of Kuffra, they swore that they will keep up the fight until the french flag will flies over the Cathedral of Strasbourg.