The emperor had no idea his beloved Consort Li was cheating on him. He doted on his sixth son and personally stripped his own son of his position.
Xiao Qian lived another life. You, you sixth son, still want to frame me, make Father strip me of my position as crown prince, become emperor, and unify the realm? Dream on!
And you, Father, what will you do when you learn that your beloved Consort Li has been cuckolding you? When you discover that the sixth prince you've doted on since childhood isn't your son? When you realize you oppressed your own flesh and blood for that bastard? Will you be so furious you'll cough up blood?
In this life, he asked to guard his mother's tomb. When the Northern Di cavalry returned and trampled their way to the capital, poised to breach its walls, only then did the Great Qian Emperor and his entire court remember the man who had once fought across the realm for Great Qian, expanding its territory and earning the title of Great Qian's God of War. The Great Qian Emperor: "My son, come save me at once! You want the throne, right? I'll pass it to you immediately." The entire court: "Your Highness, if you don't come out of retirement, Great Qian is doomed!" The city's nobles knelt and begged: "Your Highness, please come out of retirement!" Xiao Qian: "Get lost!"