The history of the Japanese Imperial Family is ridden with virtually powerless emperors who did almost nothing of importance. But Go-Sanjo (1034-1073) is one of the few exceptions.
By the mid 1000s, the mighty Fujiwara clan had dominated Japanese politics for centuries. Their secret was their ability to produce numerous daughters, combined with their success in having those daughters married to sitting emperors, and those daughters having uncommon success at producing healthy heirs to the throne.
Because in Japanese tradition children were raised in their mother's household, and rarely saw their real fathers, this meant that for several hundred years, future Japanese emperors had been raised from birth by their Fujiwara mothers and Fujiwara grandfathers, and were thus very liable to do the bidding of the Fujiwara clan once they assumed the throne.
Just to be sure, the Fujiwara instituted a system of regency, whereby the Fujiwara clan head made all political decisions while the emperor frolicked about and wrote poetry or whatever else. And just to be extra sure, the Fujiwara eventually made a practice of demanding that all emperors abdicate while still children, in favor of yet another child emperor, on the wise understanding that a 5-year-old emperor is much more likely to listen to what his mother and grandfather say than a 20-year-old emperor.
However, in the year 1068 this clever system the Fujiwara had concocted to secure their rule of Japan finally came crashing down. A perfect storm of infighting among several factions of the Fujiwara family, a recent inability of the Fujiwara to bear enough daughters, and the further inability of what few daughters they did have to bear sons to emperors, led to a situation in which the only acceptable heir to the throne, following the death Emperor Go-Reizei, was one Prince Takahito.
But there were two huge problems with Takahito, from the perspective of the Fujiwara family. First, his mother was not a Fujiwara, so he had not been raised in the Fujiwara house and bore no loyalty to the Fujiwara clan head, and second, he was already well into adulthood, being already in his mid 30s when he took the throne. The emperor who would later receive the reign name of "Go-Sanjo" therefore became the first Japanese emperor in 200 years who ruled as well as reigned.
Although a regent (kampaku) was dutifully appointed from the ranks of the Fujiwara, as tradition demanded, Go-Sanjo was determined to rule on his own, so he was not at all content to just sit back and let the regent make all the decisions. He immediately set about attempting to strengthen the political and financial situation of the Imperial Family, which had greatly deteriorated during the long years of powerless infant emperors under the thumb of the Fujiwara.
Most notably, in 1069, Go-Sanjo established the kirokujo, or "records office," to scrutinize and possibly revoke the charters of the shoen land grants which the nobility had been freely concocting for years in order to appropriate land away from the Imperial Household. Although the kirokujo was not immediately very successful at overturning the bogus charters, it would eventually be used with somewhat greater success by later emperors to return lands to the Imperial portfolio.
Frustrated in most of his efforts to rollback the shoen system, Go-Sanjo finally decided that if you can't beat them, join them, and began taking steps to shore up Imperial finances by having the Imperial Family acquire shoen of its own via loyal proxies (a strange situation by which the Imperial Family was essentially stealing land from itself in order to secure a more steady income).
At last, completely frustrated in most of his efforts to rule effectively due to interference at every turn from the nobility, Go-Sanjo abdicated the throne in favor of his son, who would become one of the most powerful emperors in Japanese History, Emperor Shirakawa. Go-Sanjo's abdication was his real masterstroke, as it allowed him to control who succeeded him, letting him bequeath the throne to the loyal and able Shirakawa, and it also allowed him much greater freedom of action to manage the Imperial family from his position as a retired emperor, because he was away from the endless ritual and prying eyes of the court. Acccordingly, many historians use Go-Sanjo's abdication in 1072 to mark the origins of the insei system of indirect rule by retired emperors, by which the Imperial Family was once again able to rule over Japan.
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