I want to know the answer to the following question:
Who threatened the authority of Japan's central government?
All the background I'm provided with is that "As early as 939 A.D, Taira no Masakado threatened the authority of the central government, leading an uprising in the eastern province of Hitachi, and almost simultaneously, Fujiwara no Sumitomo rebelled in the west.". Is that enough to answer the question?