My question is that while numbers of historical empires such as the Imperial Britain or Ottomans were always seeking to expand their territory by conquering nearby empires, why didn't Ming dynasty, one of the most prosperous eras of medieval China, conquer nearby smaller empires? Very primitive question so would be keen to elaborate and clarify some parts.
Edit: all the answers were fascinating. Thank you so much!
First of all, a bit of clarification – the Ming waged several wars against Vietnam – whilst I’m not entirely qualified to write in depth about it, see the Ming–Hồ War and subsequent control of Vietnam by the Ming. To more directly answer your question, it can be conversely asked, why would they invade and conquer their surrounding neighbours?
Through sheer lack of necessity, through the acceptance (or rather, for lack of a better word, integration) of other cultures by bringing them into the fold of Chinese culture, or simply through the Mandate of Heaven and traditional values that laid the foundation of solidarity in Chinese administration, the empire was able to maintain its basic outline and structure despite many shifts in government rule. Being ruled by different ethnicities (the Han of the Song dynasty, the Mongols in the Yuan, then the Han again in the Ming, and then later the Manchus of the Qing) surely didn’t help in cementing their power amongst the Asian region, and yet somehow it did. The spread of Chinese culture, the sinicisation process, and a significant number of court administrators were core to the preservation of a rather contiguous civilisation, and in a way, answers your question. By establishing common ground with the ‘barbarians’ bordering China to the north and west, as well as giving an impressive display of solidarity and power to other kingdoms, China was engaged in a peaceful tributary system with its neighbours. The benefits are quite clear – a source of income and supply without dedicating large amounts of resources and time (not to mention the destruction of lands) in having to physically invade, conquer, and pacify smaller empires.
Already this stands in stark contrast to the situations that both the Ottoman and British Empires faced, whereby unlike China they were either unable to maintain unbroken peace or otherwise faced a pressing need to expand. To touch on the first point on maintain civil order – the bureaucratic system of China (despite, yes, steering off that path as seen during the Three Kingdoms era) meant that officials and administrators were undeniably useful in allowing culture to spread, and thus via proxy, order and peace. Of course, asking why this didn’t happen in Europe opens you up to so many different reasons and what-ifs (which really would warrant another question on its own), but suffice to say that the same integration and ‘mixing’ of cultural values did not occur to as deep an extent in European/Near East civilisations as it did in China.
This also ties in to the need to expand as a way of securing the borders and preventing incursions of external powers – not to mention that the idea of simply invading another empire is somehow an easy task. Again, a hegemonic tributary system established by China was much more favourable, and this idea of not having to invade is compounded by the fact that, comparatively, China’s mainland was rich in resources – whether gathered through the assimilation of cultures or simply by virtue of geography, they had little need to actively look outwards for resources. This was also one of the reasons for the beginning of the Opium Wars and China’s insistence in not willing to trade substantial amounts of their own goods to the British for opium. Similarly, the expeditions of Zheng He served a very important role of both cementing the power and authority of Ming China as well as setting up and maintaining important tributes and monuments.
Whilst the Ming didn’t invade and conquer many of its surrounding empires, it definitely did commit many resources to strengthening its northern frontiers against surrounding barbarians who, despite tributary relationships, were always a threat. Thus, through a combination of several factors such as that of administration systems, cultural and traditional values, and geographical necessity, the Ming did not actively seek to expand outwards as their European and Near East contemporaries would have. It would be important to note too that this systematic treatment of their neighbours is seen in varying degrees across many dynasties of China, and is not limited just to the Ming (although in many ways and at brief glance, it can be concluded that way).
For further reading specifically on the Ming, I’d recommend John Fairbank’s “China – A New History” (specifically chapter 6, on the Ming government), Jonathan Spence’s “The Search for Modern China” (specifically the first chapter on the Ming), and Patricia Ebrey’s “The Cambridge Illustrated History of China” (specifically the 7th chapter on the Ming). Outside the scope of the Ming dynasty, these books are still great introductions and reference books to China as a whole, and they should also be more than enough to answer most other questions you may have (and even give rise to some more questions).
I can provide some detail on the situation in Korea, which was being ruled by the Goryeo/Koryŏ dynasty when the Ming dynasty was founded in China. Goryeo officials were divided on whether to align themselves with the Yuan or the Ming. In 1388, a Goryeo expedition was sent to attack the Ming (edited to remove the statement that this was to support the Yuan, since this was after the major Yuan-loyalist forces had already been defeated in the region) in the Liaodong peninsula under the orders of a Yuan-aligned general. However, the Ming-aligned general who was commanding the force turned around at the border and returned to the capital to stage a coup, and in 1392 overthrew the Goryeo and himself became king of the new dynasty, Joseon/Chosŏn. So Joseon was from the beginning a friendly regime to the Ming, which recognized the Ming as the rightful ruler of China and were willing to play the part in the traditional tributary system of East Asia. For their part, the Ming, who were fighting Manchurian forces loyal to the Yuan, were in no position to make further enemies.
Geography provides a large part of the explanation for why Chinese forces were never able to make lasting conquests in Korea (apart from short-lived commanderies in ancient times), let alone Japan. Korea is a rugged peninsula which will stretch the supply route of any invading force from China (as an example, a disastrous series of campaigns into Korea by the Sui dynasty of China in the 600s contributed to its demise). Even the Mongols needed 30 years to stamp out the last bit of resistance from Goryeo, which unlike China survived as a client state under Yuan overlordship. From the Yuan point of view, it was convenient to leave the Goryeo state intact as a regional counterweight to rival Mongol factions in Manchuria who could always threaten the central Yuan court.
This brings us to the fact that Manchuria, mostly populated by Jurchen tribes, served as a useful buffer between China and Korea. The Ming and Joseon could each expand their own borders at the expense of the Jurchens or vie for influence over the Jurchens without directly confronting each other. The Ming did eventually win overlordship over most of Manchuria (albeit briefly), but unlike the Yuan they never managed to incorporate it fully into the empire. The Jurchen chiefs, nominal vassals of the Ming, ran their own affairs in Manchuria. Eventually the Manchus, descended from a branch of the Jurchens, would establish the Qing and overthrow the Ming in China.
Japan on the other hand was doubly protected from continental invasions by Korea and by the sea. The only time that continental forces were able to invade Japan was when the Mongols had finally pacified Korea, and the joint Yuan-Goryeo naval expedition to Japan was destroyed by kamikaze, the divine wind. The fact that the Mongols were bogged down in Korea for thirty years and had no experience in naval warfare to begin with saved Japan from the full brunt of the Mongol onslaught. Ryukyu was similarly protected by the sea.
From an economical standpoint, the Ming did not have the economic resources to sustain massive military expenditures needed for conquests. Owing to his background as a peasant, the Hongwu emperor set down the lowest land tax in Chinese history. In the early Ming, the land tax was 0.0335 dan (piculs) per mu (acreage).
The Ming collected tax revenue from a variety of sources - grain/rice, cloth, iron, tea, silk, etc., but the two biggest sources of revenue were the land tax (paid in grain) and the salt monopoly.
Da Ming Hui Dian Wanli 7 (1578):
In 1522, 0.45 tael of silver was equivalent to one dan of grain, and in the Wanli reign, one tael of silver was around 2 dan of grain, so using that exchange rate, the combined income for grain and salt in 1578 would have been 14,638,546.5 taels of silver.
I don't have the military figures for 1578, but in 1593, according to the Hui Jilu and Wu Beizhi:
Total: 651,665 soldiers with 279,158 horses, oxen, and camels
Expenditure: 7,154,630 taels of silver and 1,900,000 dan of grain.
1,900,000 dan of grain equals roughly 900,000 taels of silver.
So if we assume that the revenue from grain and salt in 1593 was also somewhere around 15 million taels, that would mean almost half of the Ming's revenue would be spent on maintaining the military in peacetime. During wars, this figure would be even higher.
It is also important to keep in mind that tax revenues decreased over time for a variety of reasons. So the revenue figures you see here were much lower than during Yongle's reign, which was arguably the golden age of Ming military.
Obviously this is a very basic estimate, and I'm not counting other sources of revenue, but this should give you an idea of how expensive military upkeep was during the Ming and why they could not sustain prolonged periods of warfare. There were officials who opposed the Korean expedition in 1593 precisely because they were afraid of draining the treasury.
Edit 1: The Ming's central treasury was known as the Taicang (the largest of several treasury vaults). Harry Miller's text State Versus Gentry in Late Ming China includes a table drawn from Chinese historians showing that that between the periods of 1587 to 1593, the Taicang had around 4 million taels of silver in revenue and 5 million taels of silver in expenditures, a deficit of nearly a million taels. James Tong estimates that from 1548 to 1617, between 60-80% of expenditures from the Taicang were military expenditures.