Complex sentence: Thus, in 1921 when the Emperor Tashio's incapacity became so acute he could no longer reign, Crown Prince Hirohito became regent while stripped of his duties and powers, Tashio remained on the Chrysanthemum throne.
Split into simpler sentences: Thus, in 1921 when the Emperor Taishō's incapacity became so acute he could no longer reign, Crown Prince Hirohito became regent. While stripped of his duties and powers, Taishō remained on the Chrysanthemum throne.