When Emperor Xuan died, Northern Zhou's successor state, Sui China, had been attacking Chen, but upon hearing about Emperor Xuan's death, Emperor Wen of Sui (Yang Jian) decided that it was improper to attack a state that had just lost its emperor and withdrew his forces. He also sent ambassadors to mourn Emperor Xuan's death, and his letter to Chen Shubao referred to himself by his personal name—a sign of humility. However, Chen Shubao's return letter included the sentence, "May it be that when you govern your state, all things can be well, and that between heaven and earth, there will be peace and quiet." The sentence was viewed as arrogant and condescending by Emperor Wen and his key official Yang Su and made them displeased. Despite this, however, for the next few years, the states exchanged ambassadors often and generally had peaceful relations, although Sui's Emperor Wen was gradually building up military power on the Yangtze and planning for eventual attack against Chen.