There are several factors that impacted Chinese expansion southward. Some environmental, some cultural, some political, and some just “the way the dice fell.” And while it’s not accurate to say that they “never” tried, there are good reasons for the Chinese Empire to have never officially expanded (much) into Indochina.

Environmentally, moving from the temperate region of the Chinese “heartland” – primarily the regions of Hebei, Shanxi, and Shandong between the Yellow and Huai River Valleys – southward past the Yangtze and beyond into what is today southern China proper has been very challenging. The Yangtze, since the time of the Three Kingdoms period of the 3rd century, has served as the semi-official dividing line between north and south, because things tended to go very wrong, very quick for northern armies seeking to cross it. One of the big factors – and one that historically tends become ever more exacerbated the further south one seeks to thrust with their army of northerners – is that of low latitude diseases such as cholera, malaria, dysentery, and gangrene. Records abound from the Han, to the Tang, Song, Yuan, all the way to the end of the Qing of imperial armies reported as being as large as in the hundreds of thousands being dispatched to subdue a rebellious vassal king in Annam, or take control of Champa, or the like… only to see it come staggering back months later at 10% strength, all of its officer corp dead, and reporting that they never made any contact with an enemy force, but the men started dropping like flies day by day. This was furthered by the terrain and sheer forestation of the region, which slowed down any advance, concealed defenders, allowed for ambushes and traps to be constructed with ease, and further demoralized those troops unfamiliar with the region. This makes Indochina one of the very few non-island regions targeted by the Mongol khans (in this case, Khubilai) that subsequently failed outright in achieving its goal of subjugation. From Morris Rossabi:

Khubilai then turned to Champa (then roughly equivalent to modern South Vietnam), whose king, Jaya Indravarman VI, was hostile to the Mongols. The monarch of Champa refused to accompany a tribute mission to the Yuan court and rebuked several of Khubilai's envoys. In 1281, Khubilai responded by sending Sodu, one of his leading officials in Canton, on a punitive expedition against him. Sodu, commanding a force of one hundred ships and five thousand men, landed on the coast of Champa, but the guerrilla warfare that the king employed prevented the Mongols from making any headway. Khubilai then decided to seek the cooperation of the kingdom of Annam (present-day North Vietnam) against its neighbor to the south. Its king, Tran Thanh-ton, although he had sent embassies to Khubilai's court, was not, however, eager to have Mongolian troops cross his land to reach Champa, and so he too fiercely resisted the Mongolian forces led by Sodu and Khubilai's son Toghon. Guerrilla warfare, heat, and disease took their toll on the invaders, and in a decisive battle fought at Xuming on the border with Yunnan in the summer of 1285, the Mongols were defeated and Sodu was killed.

A second expedition, led by Nasir al-Din and Khubilai's grandson Esen Temiir, later joined by Toghon, campaigned in Annam in 1286 and 1287 and even reached Hanoi but was forced to withdraw because of the heat and the unfavorable environment, much to Khubilai's anger. The Annamese king and the king of Champa recognized, however, that the Mongols would continue to plague them unless they offered pro forma acquiescence to the great khan. They therefore began to send tribute, and the Mongolian expeditions against them ended in the late 1280s. In 1287, Khubilai sent one more expedition, led by his grandson Esen Temiir, against the "insolent" king of Pagan. The campaign was successful; the king, Narathihapate, was overthrown; and his successors offered tribute to the Mongolian court. The expedition, however, had been costly, and it is difficult to justify the Mongolian expenditures.

That leads us to the political and cultural considerations that need to be taken into account when looking at this question. As seen the quote above, what both the Yuan monarch and those of dynasties both before and after were seeking not just in Indochina, but across all of their respective border regions, was not necessarily outright conquest – which was always uncertain, and ever-costly – but rather they’d typically seek the submission of neighboring rulers via tribute and vassalization. Two especially longstanding examples are those of Korea and of North Vietnam. Both of these regions were considered to be within the Chinese “sphere of influence” at the height of virtually any dynasty… and yet for that, there were few if any real efforts to effectively stamp out their individuality or cultural autonomy. Now, this is certainly in part because both the Koreans (especially the Goguryeo) and the Viets put up such tremendous resistance to over Chinese military invasions that in some cases they caused the outright collapse of entire Chinese dynasties in the process, but there was also the recognition on the part of most imperial courts that so long as appropriate tribute was paid and the correct level of deference accepted by the local kings, pushing the issue further was not worth the hassle.

Especially in the south, this laissez faire political attitude was compounded by its sheer physical distance from the center of Chinese power – be it in Luoyang, Chang’an, Kaifeng, Hangzhou, or Zhongdu/Beijing. Though we often think of ancient empires in much the same way that we think of modern nation-states – complete with fixed borders that we can color in on maps – the conception at the time was rather different. Rather than being a “solid zone of control that goes from this border, to that border, and then stops at the line… Chinese imperial understanding of their power are more similar to iron shavings in the presence of a magnet – the magnet, of course, being the imperial throne itself. Power radiated out from the throne, and was picked up and amplified by regional governors… but as one departed the loci of powerand authority, that magnetism became less and less obvious. So in many respects, there was less a border, than an eventual “bow-shock” in which one found themselves beyond the socio-political pull of that central authority any longer.

As that “magnetic power” waned with increasing distance from the throne itself, so too did much in the way of Chinese official interest altogether. If it wasn’t at or near the beating heart of the Imperium, it wasn’t really worth thinking about. It’s no accident that the two southernmost provinces – Guangdong and Guangxi (well ok, one province and one autonomous region) were named that Western and Eastern “Expanses/Wastelands” – that was largely the conception of the region when it was officially incorporated into the (then fractured) Chinese state during the Three Kingdoms. In a similar vein, the province of Hainan Island was largely used by various administrations as the place one sent their political enemies careers to die. As the furthest-flung place away from the capital and the throne, it may as well have been the edge of the universe as far as court officials were concerned.

But now for the twist – how China actually did at least partially colonize the region! Or more specifically, Mongol Yuan overlords “encouraged” their subjects to do so. Markedly unlike their Han and even European counterparts, the Mongols – as we’re all no doubt well aware – were very keen on continuous expansion, and near as much with the process of global trade. Despite earlier trade routes being primarily overland, such as the long out-of-use Silk Road, or as in the case of the earlier phases of the Mongol conquests shadowing war routes, by the time of Khubilai the Mongol leadership had come to realize the efficacy of water routes versus land routes in terms of shipment of large amounts of goods. This encouraged shipbuilding and the financing and expansion of port cities that had usually been more ignored by the Chinese emperors in favor of the inland power centers and the poo-pooing of trade and commerce altogether. From Weatherford, “ Khubilai Khan made the strategic decision to transport food within his empire primarily by ship because he realized how much cheaper and more efficient water transportation, which was dependent on wind and current, was than the much slower land transport, which was dependent on the labor of humans and animals that required constant feeding. In the first years, the Mongols moved some 3,000 tons by ship, but by 1329 it had grown to 210,000 tons.”

This new focus on seafaring ships for the purposes of conducting peaceable trade, rather than ferrying armies, resulted in those ships and their crews encountering many new (at least to them) marketable goods. Southeast Asia, they’d come to find, wasn’t just full of poisonous traps, guerrilla bandits, and hemorrhagic fevers… but also goods like sugar, ivory, cinnamon, and cotton that were near impossible to produce in the north. “To expand the trade into new areas beyond Mongol political control [which had by this point reached its zenith], they encouraged some of their vassals, particularly the South Chinese, to emigrate and set up trading stations in foreign ports. Throughout the rule of the Mongol dynasty, thousands of Chinese left home and sailed off to settle along the coastal communities of Vietnam, Cambodia, the Malay Peninula, Borneo, Java, and Sumatra. They worked mostly in shipping and trade and as merchants up and down the rivers leading to the ports, but they gradually expanded into other professions as well.”

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