Complex sentence: He is also considered historically important by many Koreans since he once led Tang army in the Battle of Talas, despite being the commander of losing side, helped spread the paper and compass to the rest of the world.
Split into simpler sentences: He is also considered historically important by many Koreans since he led the Tang army in the Battle of Talas. Despite being the commander of the losing side, the battle helped spread paper and compass to the rest of the world.