Well there’s a lot to unpack here, but I think the central point to note is that China didn’t really stay unified after the Second World War, in fact it fought an incredibly bloody, brutal, and divisive civil war between the 1945-1949. Historically speaking to give a brief rundown of modern Chinese history, the CCP did not emerge from the Second World War as rulers of China, but only after a much overlooked but very significant civil war .

China was ruled by a monarchy, under the Qing dynasty until 1911, when a broadly disorganised and spontaneous revolution swept the Qing from power and established the Republic of China. The ROC was initially led by the Tongmenghui, a revolutionary party led by Sun Yat-sen, but was quickly replaced by a militaristic dictator, Yuan Shikai. After Shikai’s death in 1916, China splintered into a central government (the Beiyang Government) and a number of regional strongmen known as the warlords. The Tongmenghui now reformed as the Koumintang, a nationalist party who attempted to reunify China under central control. In doing so, they formed an alliance with the Chinese Communist Party, the CCP, which became known as the First United Front.

Piece by piece the coalition of nationalists (KMT) and communists (KMT) managed to regain control of China, expanding their control both militarily and diplomatically from their stronghold in southern China, to implicit control of most of the country. However, I’m 1927, the KMT leader, Chiang Kai Shek betrayed the communists by launching an attack on then known as the Shanghai Purge, in which CCP officials were ambushed and executed in large numbers. This then spread into a much wider attack on the communists throughout China in what came to be known as the ‘White Terror’.

This marked the end of the First United Front, and the beginning of a civil war between the CCP and KMT. The struggle would last for years, and the CCP as underdogs would almost be defeated many times.

However, in 1936, the two sides reconciled temporarily under the threat of Japanese aggression. This lead to the Second United Front, and saw the invasion of China by Japan the following year. The two sides nominally fought together against Japan, but the two sides quickly fell apart and open conflict resumed in 1945 after Japan’s surrender. What followed was an incredibly bloody, and brutal civil war in which the CCP, from an initial position of weakness, eventually defeated the KMT in 1949 and was able to establish itself as rulers of the news People’s Republic of China.

So from what you can see there, is ultimately China wasn’t unified before the war, and wasn’t afterwards either, having fought a devastating civil war to reunify that was only concluded in 1949.

However, to question why the CCP was able to hold such a wide territory is another matter. For one part, the CCP by 1949 had a very large, experienced, and well equipped military. But, more importantly the CCP offered a political platform that promised to provide positive change for the mass of China’s population, in comparison the KMT offered much and provided little. As such, many genuinely supported the CCP. Meanwhile, the KMT has proved itself an ineffective and corrupt ruling party, and had become unpopular, even within its base.

Sadly though, as noted by Dikötter, the realIties of liberation would prove somewhat less than ideal.

It’s often difficult to try and compare two countries respective historical journeys, in the fashion you’ve asked with your question, for while some regions found instability in the aftermath of war, the Second World War for China was really little but an interlude in a much longer struggle for mastery of China which had been going on since at least 1911.

There’s a number of good books and authors about Chinese history if you’re interested. Jonathan Fennel’s history of modern China is very good and well written for non experts, and Jonathan Spence’s Search for Modern China is also highly highly recommended. Both cover the entire period from the Qing to the Maoist era (and beyond). Further there’s a number of good writers: anything by Kirby, Eastman or Zarrow is good, while Westad and Pepper write very good books about the 1945-1949 civil war.

Edit: For a far more in-depth discussion of the 1937-1949 period see u/Enclavedmicrostates brilliant answer below.

A common but likely apocryphal anecdote relates that when a Japanese diplomat tried to apologise to Mao Zedong for the Second Sino-Japanese War, Mao responded that in fact, he should be thanking the Japanese for facilitating the Communist victory over the Nationalists! While this almost certainly didn't happen, it still relates a truth of a sort: the war had essentially done the work of destroying the Communists' enemies for them.

Helpfully, you do elaborate quite a bit, so your question provides quite a useful framework for the answer to follow. Firstly, we need to clear up a major misconception:

There isn't a unifying language or culture that would hold people together

While it is true that the spoken languages of China are very diverse (I should know as I speak one natively and know bits of another), the Nationalists had been promoting Beijing-dialect Mandarin as a lingua franca in the 1920s and 30s, and the written language operated on a unified system based on classical Chinese and later Beijing Mandarin. While the idea of a unified Han Chinese 'culture' has always been rather nebulous, there was nonetheless a strong ethnic-nationalist consciousness left over from the waning years of the Qing that served to bind together the vast majority of the Chinese population.

However, the China of post WW2 seemed to have had a strong grip on all the lands, despite seemingly having none of those advantages, as in, it's army was weak, it's economy was garbage, it didn't offer a lot of local independence and it was technologically behind by a few decades compared to most other global powers.

Well, China immediately after WW2 was the Nationalist Republic of China, which didn't hold onto that land, because four years of civil war ensued in which the Communists won. However, it is actually, paradoxically, kind of true that the Nationalists held onto a more united China than they had in 1937. There are two main reasons for that. Firstly, there was a general recognition that, as a relatively moderate political figure with widespread popularity, Chiang was necessary to keeping the grand coalition of Nationalists, Communists and warlords united. Secondly, the exigencies of the war had given Chiang Kai-Shek and the centre-conservative wing of the Kuomintang the upper hand in its internal power-struggles: the warlords had been driven into the arms of the KMT by the Japanese, while the radical wing had been severely weakened by the defection of its figurehead, Wang Jingwei (Wong Siu-Ming), to the Japanese and his formation of a collaborationist government.

The one problem was the Communists. Their strategy of predominantly guerrilla warfare against the Japanese had meant they had taken relatively few losses, while broadening their recognition, and by extension recruitment base, substantially. While a KMT campaign had driven the CCP out of its core regions in North China by 1947, a series of CCP offensives with Soviet support, beginning from the heavy industrial centre of Manchuria, was able to let the CCP take over most of China by 1949. This was not achieved by having a 'weak army' – the People's Liberation Army numbered somewhere in the realm of 4 million by the time the Nationalists evacuated to Taiwan, and over 1.3 million Chinese 'volunteers' were committed to the Korean War in 1950-3. While the economy was weak on the whole in relativel, global terms, the CCP held on long enough to rectify that. Local autonomy was in one sense actually rather substantial, as the anti-landlord campaigns took the form of mass action, largely by tenant farmers, which gave ordinary peasants a chance at direct political activity which had not been possible under the Republic, which operated on an electoral system with limited franchise. State-organised mass participation was still participation, and compared to the apparent malaise of the Nationalists, who in 1938 had deliberately flooded sections of the Yellow River for little apparent gain, the CCP's brand of publicly-serving Marxism-Leninism offered an attractive alternative.

I perfectly understand how China is able to maintain it's territory nowadays, but I can't make sense how it was able to be unified in the mid 20th century under the CCP, granted, with fewer lands than those around the glory times of the Qing dynasty, but not significantly so (when you consider quality of land, not quantity alone).

As noted, the CCP had few enemies and broad support from the agrarian labour class, albeit mainly concentrated in the Yellow River basin to begin with. Both propaganda and policy helped galvanise public opinion in their favour, and was helped by the relative chaos of the preceding decades. Of course, CCP rule took a while to consolidate, but there just doesn't seem to have been a desire for the core regions of China to break away, only the Qing conquests of Tibet and Xinjiang (Manchuria itself having become almost entirely Han thanks to migration). All the regional power concentrations represented by the warlords had been first absorbed into the KMT and in turn obliterated by the CCP. The Communists promised ordinary peasants redress against their traditional oppressors, the landlords. And, in the end, they held onto power strongly enough to survive the tumult of the Great Leap Forward, the Cultural Revolution and the Gang of Four, until the beginning of liberalisation under Deng Xiaoping.

As for sources, there are a couple of more polemical Western authors out there like Frank Dikötter, but I would heartily recommend Rana Mitter's A Bitter Revolution or Hans van de Ven's China at War as good perspectives on the period in question, albeit the former having a broader view than the latter.

I'd like to take a slightly different approach to answering this question and take a look at how things played out in the periphery, focusing on Xinjiang. Xinjiang had been under the control of various different warlords since the fall of the Qing Dynasty in 1911 who really only paid lip service to the Republican government(s), while Outer Mongolia and Tibet would become independent states. However, unlike in Mongolia and Tibet, Han elites would continue to run the show in Xinjiang, meaning that in the words of the historian Justin Jacobs, "for the duration of the Republican era, we might say that Xinjiang was one of China's last remaining colonies."

​

By the mid-1930s, Xinjiang had become a Soviet satellite under the warlord Sheng Shicai. It's worth noting that it's unlikely that Stalin sought to outright annex Xinjiang, since we have telegrams from him saying that Sheng's talk of "Sovietizing" Xinjiang was "alarming." Returning to the topic at hand, the KMT had spent the years since the outbreak of the Sino-Japanese War re-establishing control over the northwest. By 1941, they had managed to bring the Ma family of warlords in Gansu, Qinghai, and Ningxia into the fold. (Map for reference) The following year, Sheng Shicai would double-cross Stalin, expelling Soviet advisors and aligning himself with the KMT. They had assured him he would remain in power and the finance minister gave him a large bribe paid him $100,000 "under the pretext of facilitating his work as a provincial governor." (Lin, 55) The money was just resting in his account, that's all.

​

This leads to a couple of questions: First, why was the KMT spending resources on the interior? Second, why would Sheng Shicai make the generally bad decisions to not only double-cross Stalin but also trust Chiang Kai-shek?

​

As /u/EnclavedMicrostate discussed in his answer, the war with Japan is a major factor. Control over Xinjiang would not only provide a lot of territory for the Nationalists, but it would also offer potential supply routes to make up for the loss of sea access and serve as an alternative to the Burma Road. Although "technical and geographical issues" prevented the development of large-scale supply routes through Xinjiang, it did serve as a base for extending political influence into other areas, particularly Tibet. (Lin 65) The KMT also saw Xinjiang as "a springboard into the Middle East," from which diplomatic efforts could be launched that could offer China access to the region's oil reserves. (Lin 69) For his part, Sheng thought that the Soviets were going to lose their fight against the Nazis, while Chiang's odds now seemed better given US entry into the war.

​

Sheng would come to regret his choices pretty damn quick: Nationalist troops and several thousand KMT cadres poured into the region, rapidly starting to build up colonization and infrastructure projects to use the area's resources for the war effort. In 1944, Sheng tried to switch back to the Soviets, who flatly rejected him. KMT control had become firm enough that Chiang was able to remove Sheng as governor and appoint his pick in September. The reassertion of central control "provoked a backlash among ethnic minorities," leading to several uprisings by Uyghurs and Kazakhs, most notably the 1944 Ili Rebellion carried out by Uyghurs under the banner of the East Turkestan Republic. (Lin 70-71) These revolts generally enjoyed Soviet backing, but this support was largely given in the pursuit of extracting other concessions from the Chinese government. (Wolff 283-285) I would also like to caution against calling these revolts nationalist, however. If you have access to JSTOR, Jacob’s 2010 article on Osman Batur linked below gives a pretty good explanation that narratives of Batur in particular as a Kazakh nationalist hero are later posthumous inventions.

​

The Ili Rebellion “may have paradoxically succeeded in doing far more to integrate the Xinjiang region and its people into China proper (neidi)—and even more crucially, into the greater national psyche of a Chinese republic—than its leaders ever could have imagined.” (Jacobs 2008, 549) This is largely because of the efforts of General Zhang Zhizhong, who was dispatched to deal with the revolt. After working out a power-sharing agreement with the ETR and enacting several reforms in the province, he embarked on a major effort to ensure that Xinjiang would be popularly perceived as a part of China. This would be achieved through the publication of the *Tianshan Pictorial* and a tour of China Proper by the Xinjiang Youth Song and Dance Troupe. I don’t want to get too into the weeds here, so if you want more info (and have access to JSTOR), the Justin Jacobs article from 2008 covers this at length. Ultimately, these efforts managed to “maintain the perceived territorial integrity of” China, even as it was engulfed in an intensifying civil war. (Jacobs 2008, 546) From this point on, Xinjiang would essentially remain under central government control. However, problems would flare up again after Zhang’s removal as governor in 1947, with the ETR rebelling again in 1948.

​

But how did the CCP end up in charge in Xinjiang? Local Han elites were very willing to defect, and most of the Nationalist leaders in the province defected to the CCP in September 1949, with the remainder fleeing to exile in Taiwan via India. Crucially, one of these defectors was Zhang Zhizhong, who provided pivotal knowledge about the province. This information was made all the more valuable since the remnants of the local Communist Party branch had been all but eradicated in a purge carried out by Sheng Shicai after his 1942 defection. Zhang would “continue to play a key role both in the political integration of Xinjiang during the early years of the People's Republic of China and in the subsequent formulation of the Communist state's post-1949 frontier minority policies.” (Jacobs 2008, 548-549) Without the defection of Han elites, “the Communist takeover of Xinjiang would have been more difficult, and in all likelihood, more violent.” (Lin 73) Meanwhile, the ETR’s leadership was pressured into negotiating with the CCP, but most of its leaders “died in a mysterious plane crash near Irkutsk” while traveling to the PRC’s proclamation in Beijing. (Wolff 284) Nonetheless, claims by PRC historians that Xinjiang was ‘peacefully liberated’ are, to put it bluntly, full of it, much like most of the official PRC narrative regarding Xinjiang. The PLA played a key role in consolidating power and quashing local autonomy, and “no fewer than 120 battles were fought over a two-and a-half-year period.” (Jacobs 2010, 1300).

​

Sources:

Lin, Hsiao-Ting. 2008. From Rimland to Heartland: Nationalist China's Geopolitics and Ethnopolitics in Central Asia, 1937-1952. The International History Review 30 (1) (March 2008): 52-75, www.jstor.org/stable/40109957

Jacobs, Justin. 2010. The Many Deaths of a Kazak Unaligned: Osman Batur, Chinese Decolonization, and the Nationalization of a Nomad. The American Historical Review 115 (5) (December 2010): 1291-1314, www.jstor.org/stable/23308070

Wolff, David. 2011. STALIN'S POSTWAR BORDER-MAKING TACTICS: East and West. Cahiers Du Monde Russe 52 (2/3) (April-September 2011): 273-291, www.jstor.org/stable/41708323

Jacobs, Justin. 2008. How Chinese Turkestan Became Chinese: Visualizing Zhang Zhizhong's "Tianshan Pictorial" and Xinjiang Youth Song and Dance Troupe. The Journal of Asian Studies 67 (2) (May 2008): 545-591, www.jstor.org/stable/20203377

​

EDIT: Fixed some formatting errors