I Transmigrated into a Shogun, Yet I Must Save the Southern Song

During the Southern Song, the Japanese shogun Minamoto no Sanetomo of the Kamakura Shogunate admired the Song Empire. In 1217, he hoped to sail west to the Southern Song and live as a commoner, but his ship ran aground and the journey never took place. In 1219, Minamoto no Sanetomo was assassinated. What would have happened to history if that ship had not run aground? After transmigrating into the body of Minamoto no Sanetomo in 1217, how can he return to the Southern Song, and how can he survive a seemingly inescapable fate? After losing his teacher, brother, and beloved one after another, how will he rally, support the ruler, outwit powerful ministers, and save the Southern Song? Can he even establish an East Asian front against the Mongols? Will he succeed, and will he ever return to the modern world?