Complex sentence: His diplomatic success in securing Korea in the midst of the rival Yuan and Ming Dynasties is also highly regarded, because safeguarding domestic security led the Koreans to rebuild and further discover their culture and national identity which once was threatened by the Mongols.
Split into simpler sentences: His diplomatic success in securing Korea in the midst of the rival Yuan and Ming Dynasties is highly regarded. Safeguarding domestic security led the Koreans to rebuild and further discover their culture and national identity which once was threatened by the Mongols.