How did Empress Dowagers Become so Powerful in China?

so much power was consolidated for so long in Empress Dowagers in China

The standards involved are kinda necessarily subjective, but this premise can be pretty safely doubted. I'm imagining that assumptions of this nature involve the extreme fame of the Empress Dowager Cixi relative to the other- literally innumerable- wives of the 5-600 men (however we're counting) who we'd categorize today as "Emperors of China." Then of course there was the Empress Wu, who was the one, single, only woman in the 2-6,000 years (however we're counting) of "Chinese history" to ever actually take the throne. These women are as well known as they are almost exactly because they were so exceptional, and there are people on this sub who would have a much better grasp on the specifics of their careers than I would.

There was a tiny group of women, and two in particular, throughout the entirety of these however many thousands of years, who are usually understood to have been at something in the neighborhood of the Cixi level of power. The two main ones being the Empress Lü, wife of the Han founding emperor Liu Bang; and the Empress Liu, regent of the Song Renzong Emperor. And that's pretty much it for women in actual positions of power as far as the official record is concerned. There are a great many stories from 野史/'unofficial histories' and similar either partially or fully fictional texts of women obtaining untoward levels of influence over emperors or other powerful figures, often through titillatingly scandalous means. Women at certain points in the history of some of the "Northern empires" like the Northern Wei and, especially, the Liao Empire, had a different, arguably 'higher' status than in what we'd today call the more traditionally "Han" empires of the south. But for the most part, the idea that the empress dowager, as an empress dowager, across the board and by default held enormous amounts of power isn't something that we can find a lot of practical evidence for. However, you're definitely hitting on something (that I personally think is) pretty interesting.

All the above said, China-context Women's Studies scholarship in English is often at pains to first combat the various myths of the "grotesquely oppressed/sequestered/enslaved/etc Chinese woman." Ping Yao has a very recent, convenient, and wide-ranging overview of this scholarship in her Women, Gender, and Sexuality in China: A Brief History (Routledge, 2022). In different degrees both throughout the imperial period and today, there was certainly an awful lot of essentializing involved, and a lot of finagling with concepts like yin and yang and other metaphysical notions in order to justify what was and is, clearly, a patriarchal society. Simultaneous with this, however, one's parents have ideologically always been a big deal. This includes one's mother, even if one is an emperor.

Looking broadly at the institution of the harem and the institutionalized figure of the Empress at the top of the harem, there were a handful of situations and specific conditions that could be met which would allow a mother of an emperor to exploit her ideologically necessarily dominant role as mother, and move on up from there to legitimate political power. The classic statement in English on these conditions is Lien-shen Yang's 1961 article "Female Rulers in Imperial China." A good chunk of this paper is actually a translation of 赵凤喈 Chao Feng-Chiai's essay "Empress Dowagers as Regents," from his book《中国妇女在法律上之地位》"The Position of Women in Chinese Law," which dates to the 1920s. The conditions were the fairly obvious ones of situations where an emperor becomes incapable of ruling. Either he was 1. too young, 2. too sick to attend to imperial affairs, or 3. suddenly dead or had left an edict establishing a regency involving the empress dowager. Yang does some preliminary analysis checking the list against history and finds things pretty solid. More recently, Keith McMahon in his two volume set Women Shall Not Rule and Celestial Women (2020) runs the thesis against every major empress dowager, empress, high-ranking consort, dancing girl gone big-time, and every other politically important woman from the Han on to the Qing. Then McMahon's 2013 paper "Women Rulers in Imperial China" also has a tighter focus on 'the big four.'

The main point is that while not every empress dowager went full Wu Zetian, and noting a few periods where there was an attempt to outlaw the empress dowager regency as an institution (the Ming Empire is the main example here- but it didn't quite work), in the event that an emperor unexpectedly became incapacitated, it was often entirely expected that the mother of the young heir (or the new young emperor), would step up and become to some extent politically active with her son's official affairs. There would be formal moments where court officials would request her participation in the new (or potentially coming) regency. Some empress dowagers were able to turn these requests down. Some performed their duties- again, this was something of a mother's responsibility- for the duration of the regency and then got the hell out of Dodge. Others were more ambitious, and about three or so of these women went on to become rulers in all but formal title. One founded her own empire.

I wouldn't know if this was unique worldwide, but the empress dowager regency was definitely "a Chinese thing" with established precedent and several real-world examples. However, it was first and foremost an emergency measure of ideally temporary expediency. After Wu Zetian, some empires tried to establish checks against letting something like that happen again. It did not happen again, and later figures like the Empress Liu had to put some effort into drawing hard distinctions between what the Empress Wu did and whatever it was that they were doing. But yeah, given a combination of unusual and specific factors, an empress dowager could suddenly come into a very front and center position of power. This kind of thing just didn't happen all that often, and the famous names are all exceptions to the rule.

A related note here: one of those Papers that Changed my Life™ kinda deals was Hans van Ess' 2006 article "Praise and Slander: The Evocation of Empress Lü in the Shiji and the Hanshu." In this paper, van Ess effectively obliterates the long-standing belief that Sima Qian, author of the Shiji/"Records of the Grand Historian," had some kind of wildly damning and slanderous view of the Empress Lü, who we'll acknowledge is famous to this day for very little in the way of pleasant, happy things. Ban Gu's description of her in the later Hanshu is, however, decidedly much more negative, and the surface similarities of the two texts have obscured some very significant differences in perspective between them. In van Ess' reading, Sima Qian is actually quite sympathetic to the Empress. While the Shiji doesn't shy away from describing the many gross excesses during her rise to power and effective reign (some of the details are, in fact, pretty gross), Sima Qian is usually painting her as someone forced by circumstance to go to often very extreme measures in the understandable interest of self-preservation. At most, Sima Qian, born into an old aristocratic family, holds both her and her husband in disdain for being of low birth. Ban Gu's more explicitly nasty attacks on her are based in a different kind of personal political allegiance on his part. But the interesting thing is that neither man ever blames her for being a woman. The many atrocities she's accused of are never explained by her gender. At this very early point in history, the fact that the empire had a specifically female ruler isn't being criticized on specifically so-called sexual grounds.

Back to the myths of rank oppression- it is definitely the case that warnings against letting women rule go way, way back. But in official writing (non-official writings, fictional works, etc. are a different story), there isn't much expressly 'feminine' in the various adages and arguments attacking women rulerships. Even Wu Zetian is held in rather high regard as a capable ruler in official histories of her reign. She committed a number of heinous acts in her time, but so did hundreds of male emperors. Her femaleness is rarely given explanatory force. What Ban Gu starts to get at, and contemporary scholars like van Ess and McMahon sometimes dance around, is the fact that the imperial harem- a necessary institution for maintaining the empire- was nevertheless always and forever an inherent latent security risk. Given a coincidence of highly particular factors, a sudden lack of a suitable male ruler, a sufficiently ambitious empress dowager- whether acting as formal regent or not- able to amass a sufficiently strong power base could, then, instantly overthrow an entire empire. This is what the Empress Wu did, and what everyone was afraid that every empress dowager after her who suddenly found herself in Wu Zetian's position might go and do herself. There was a built-in conflict between the ideology of imperial rule and the family as an ideological concept. A woman like Wu Zetian couldn't choose to continue the old regime because she didn't bear the surname of the imperial house. She did move to change the surname of her apparent successor, but her reign ended before anyone could see how something like that would play out. We never saw how an empire founded by a woman might work.

Assumptions of the nature of the "patriarchy" aside, there was a structural 'flaw' where, in certain rather uncommon situations, a woman could shoot straight to the top. Official writings on the issue, like Ban Gu's, are far more concerned with avoiding this immediate, instantaneous threat to the empire than with saying things like a woman, as a woman, is objectively incapable of ruling. Again, there were women rulers, but not all that many.