The House of Aisin-Gioro is one of the great imperial chameleons of the modern period: for centuries, they succeeded in presenting themselves to a huge and diverse empire as Jurchen chieftains, as Mongol khans, as Confucian emperors, and as Buddhist sage rulers, and they took control and asserted their legitimacy in different ways depending on the situation.

In the case of China, obviously the largest and most economically powerful of the Qing dominions, they had a couple key advantages before they undertook The Great Enterprise in ernest. In the 1640s, the world entered one of the most intense periods of global cooling in recorded history; not only did harvests fail, but the Grand Canal, the key artery of Chinese internal trade, dried up, preventing what had been harvested in the south from reaching the capital. This compounded the Ming government's chronic revenue shortage, and three years later, Li Zicheng's rebel forces captured Beijing, and the Chongzhen Emperor hanged himself. The Ming government in the north melted away, and after some strongarming from Dorgon, the Manchu regent after Hong Taiji's death in 1643, many Ming generals were convinced to join up under the empire to their north. Being a legitimate, functioning state made service to them much more attractive than the peasant rebels of Li Zicheng, and the passes were opened to the Manchu.

The Qing had also built up a formidable military machine; after they captured Liaoning, a Chinese province north of the Great Wall, they quickly incorporated the Han into the Eight Banners, and by the time of the Conquest, 75% of the Eight Banners were Han. The Manchu called them 'heavy troops', since they fought on foot, with spears, muskets, and artillery, rather than on horseback with the bow like their Manchu rulers. Even more Chinese troops flooded into the Qing military as large scale defections of Ming generals formed the core of the Green Standard. Taking control of the Yellow River cultural sphere of China was relatively easy when they had by far the largest, most professional army in the region.

For the Chang Jiang region and further south, though, conquest required a careful balance of carrot and stick. When the Qing took Beijing without a fight, the young Shunzhi Emperor performed the traditional offering to Heaven as a way of claiming the Mandate of Heaven, and they quickly began hosting the civil service examinations in the capital again. These were the key mechanisms for holding the loyalty of the landowning scholar elite who formed the backbone of dynastic power in China. Being able to offer the prestige that came with the examinations (hosted by the divinely mandated emperor), while also threatening horrifying violence (as happened when Yangzhou refused to surrender), the Qing made a compelling case for submission. By 1646, both Li Zicheng and Zhang Xianzhong had been killed. After capturing Beijing in 1644, it was a short three years before Qing troops took Guangzhou in 1647, and with the Yellow River, the Long River, and the Pearl River under their control, all that was left was mopping up the Zheng family of Ming loyalists, who were based in Fujian and Taiwan, which was conquered in 1683 when a succession dispute arose in the Zheng family.

The Qing built up a strong state, took advantage of opportunities presented to them, and legitimated their rule through structures familiar to their subjects.

The Great Enterprise by Frederic Wakeman is probably the definitive work on the Qing Conquest.

The Class of 1761 is a good look at how the examination system was used to hold the loyalty of local scholar-elites.

There is actually quite a bit of debate about this. The traditional explanation portrayed the Ming court as chaotic and inefficient, too busy with its petty squabbles to unite to defeat its enemies. In particular, this explanation relies on an image of the late Ming military as hopelessly corrupt, bureaucratic and ineffective, contrasted with the rising vigor of the Qing Manchu, which was organized into a highly effective fighting force by Nurhaci which combined Steppe cavalry and the new power of firearms. Add to this a dash of stereotype, that the Chinese never innovate or make new things, and you can see the explanation that tends to be written in most general history books.

Recently however, Chinese military history has actually become a thing (or at least it only recently became A Thing in western scholarship). And it has been shown that, actually, the Chinese army was pretty innovative and willing to experiment--for example there were a few attempts at making European style star forts before the design proved unsuitable for the type of warfare they engaged in. To be sure, the Ming army did have its problems, but many of these had been solved by the reforms of the Wanli emperor a good half century before the Manchu conquest. The collapse of the Ming should thus be seen as a bit if bad luck as it was hit by three major problems: external threat, internal disturbance, and ineffective leadership.

I discussed the external threat a bit above, as the Manchus were a truly effective fighting force. Still, perhaps more important than them were the internal revolts that wracked the central heartland in the decade or so before the fall of the Ming emperor. These were due to a variety of different causes and tended to have millenarian rhetoric, but broadly speaking we can look at economic causes, particularly the slow collapse of the Ming tax system. During the 1630s and early 1640s these rebellions coalesced behind two different leaders, Li Zicheng and Zhang Xianzhong. They proved highly competent and charismatic leaders, and although they did not work very well together they still managed to cause more trouble for the Ming than for each other.

The Ming probably still could have pulled through with a competent leader, but the Chongzen Emperor was not that. He has a moderately good popular reputation today because he famously chose to hang himself rather than abandon his city, as they continued to be centered onto the indefensible city of Beijing rather than retreating to Nanjing. And in general his rulings were confused and ineffective, and he did not ave a good eye for his subordinates. For example the Ming were able to get through the Great Wall due to the defection of the general Wu Sangui, who has been traditionally excoriated, but in his defense he wasn't given that many options. So as much as we try to avoid great man theories of history, we must aknwledge that, yes, the Chongzen Emperor was pretty awful.

Much of this can be found in kenneth Swope's The Military Collapse of China’s Ming Dynasty, 1618-44.