This is a very good question, and one that I think deserves a comprehensive, multi-perspective answer.

The first thing we should do is set the table. If we’re trying to understand the relationships between China, the Soviet Union, and the nascent North Korean state, we need a familiarity with the geopolitical lay of the land that we can reference when we discuss theories and narratives that will help us understand China’s role in the first decades of the North Korean state. It’s very likely this summary will be overly broad, but I would rather include too much information than risk omitting something truly valuable without realizing.

To clarify, since this is without question my longest submission to AskHistorians (or just about anything outside my academic career), the answer will be in two parts: this introductory post and a reply that focuses on perspectives of China’s support and involvement with North Korea.

China

The first thing to remember is that the People’s Republic of China wasn’t merely ‘young’ at this point in history, it was practically a newborn. The expulsion of the Kuomintang from mainland China wasn’t completed until 1949- the Korean War was already brewing at this point in history, with border clashes and military crackdowns occurring on both sides of the 38th parallel, and the war would begin in earnest less than a year later.

So when we think of the PRC in the buildup to the Korean War, we should think less of the monolithic, established Chinese state we see it today, and more of a victorious political entity in the process of consolidating control over a massive and complex country in a very tenuous geopolitical position- that is, on the very faultline of the oncoming Cold War. The second thing to note is that North Korea was of assistance to China before China was of assistance to North Korea. The Manchurian territory occupied by Japan was immediately a contested battleground in the aftermath of the dissolution of the Japanese Empire post-WWII. North Koreans, including many partisans from the long resistance to the same empire’s occupation of Korea itself, assisted the Chinese Communist Party forces in terms of equipment and manpower, helping them from 1945 onward to push the KMT forces out of their strongholds in the Northeast.

This created a makeshift strategic alliance that undergirded the ideological compatibility of the future DPRK and PRC, and it should provide some evidence that despite the importance of the machinations of the communist leadership, of Kim Il-Sung and Mao Zedong and Joseph Stalin, there existed something of a communal spirit that connected the disparate groups, sometimes down to the individual level. That a North Korean partisan would die in Manchuria while fighting the Nationalists, or that a People’s Liberation Army soldier would die in Seoul while fighting the Americans is evidence that the struggle was ideologically motivated for many of those individuals involved, a fact that the leadership of the great Communist powers would make use of for years to come. I stress this point, which many might consider a foregone conclusion, because I don’t want any of my explanations to come off as overly neat and declarative: there’s no doubt China’s grand strategic interests played a significant role in its involvement with the DPRK regime, and even more empathic voices in the scholarship admit to the tactical nature of the support floated between the two nations, but equally, there’s no doubt that the countries and militaries as represented by their people were motivated by broader cultural contours as well.

The last thing to consider for China is that the end of the Chinese Civil War as we discuss it historically was not the end of the KMT or the (original) Republic of China. Rather, they were now contained to Taiwan, where they remain today, then incredibly fearful (and rightly so) of the impending PRC invasion that would wipe out the Nationalists once and for all. But that invasion never came, and the why is fundamentally tied up in the question you asked.

Korea

I could spend another five posts just talking about the juncture in history arrived at by Korea with the end of WWII, but in order to stay on topic, I’ll be as brief as possible.

In short, the Korean peninsula’s position has long made it a geopolitical target. It has been invaded by China, by the Mongols, by Japan, and every time a sphere of influence has grown to the highest levels in East Asia, Korea has been a very real consideration in its path. At the turn of the 20th century, the Sino-Japanese and Russo-Japanese wars were the culmination of expansive designs on the peninsula, both utter calamities for the former countries that contributed in their own way to the collapse of the Qing and Russian empires, respectively. With Japan’s newly-won supremacy in East Asia came the annexation of Korea (and later, the seizure of Manchuria), not to end until Japan’s surrender to the US in 1945.

The colonial period was an extremely difficult time for Korea and its people. Stripped of their sovereignty, the once isolated peninsula was pressed into the workings of the Japanese imperial machine. Korean industry was turned towards the benefit of the Japanese, the Korean identity was systematically assaulted and the Korean people were often cruelly abused at the hands of their colonial masters. It is from this generational period that the individuals responsible for the ROK and the DPRK were forged, as the right and left flanks of the Korean nationalism that resisted the Japanese occupation, ideologically and often physically.

The takeaway from all this is a sense of who the Korean ideological leadership was on either side of the conflict at this point in history: highly motivated partisans who had fought, in one context or another, for the majority of their adult lives towards the independence of Korea and its people. Of course, the political leadership in its totality was far murkier- with many direct collaborators retaining power after the removal of the Japanese- but here we’re veering off topic, so let’s move on.

Russia

The role played by the USSR in popular depictions of the conflicts in China and Korea is often that of puppet-master, and that’s not an altogether baseless idea. Simply put, the USSR was involved with the development of the CCP and its eventual triumph over the Nationalists, despite at previous instances supporting them, and directing the CCP to support them as well. Similarly, the Soviets were deeply responsible for the formation of the DPRK as it stood on the eve of the Korean War, helping them to create and arm the Korean People’s Army, and providing them with the logistical and technical support needed to engage in the Communist reforms that gave North Korea the ideological edge in the buildup to war. During the war, however, the USSR’s role was secondary if not tertiary, so we won’t dwell too much here. The important thing is that we consider the Sino-Soviet Treaty of Friendship, Alliance and Mutual Assistance, the 1950 formalization of defensive ties and economic assistance between the two nations.

America

The US is important in these calculations for a number of reasons, but from what we’ve discussed so far, you can probably guess the first: with US victory over Imperial Japan, and the nature of the Pacific Campaign, the US inherited much of Japan’s geopolitical influence. The Yalta Conference established Russian-US trusteeship over the Korean peninsula, and the occupation force intended for the unrealized invasion of mainland Japan was diverted to the southern half of Korea. Here, the United States made a series of deep miscalculations, mostly as a result of USAMGIK’s (the newly christened occupation force) lack of preparation or expertise for the trials of Korean liberation. Most critically, the US forces in Korea failed to remove the colonial Japanese infrastructure (including Japanese officials themselves!) which led to widespread discontent. The newly empowered provisional government which would eventually become the ROK as we know it today dealt with that discontent in a truly brutal fashion, killing hundreds of thousands of leftist sympathizers in the buildup to the Korean War.

I can only touch on this a little bit. But it's been 9 hours and no one else has commented so I'll contribute what I can and hope someone who knows more can add to that.

In the 1950s when China and the USSR weren't getting along as well as before, Kim effectively alienated the Chinese, leading to them denouncing him. While he distanced himself from the USSR initially, he still maintained formal relations with the Soviets and, near the end of the 1960s when the Cultural Revolution was in full swing in China, he switched his position and — despite his disagreements with what was going on in the USSR at the time — he got closer to the USSR and further from China. This pissed Mao off, hence the denunciation.

Before this, the Chinese had the People's Volunteer Army which fought in the Korean War. Mao Zedong's son actually died in North Korea in 1950, and at least initially, was buried in Pyongyang. Militarily, in the 1940s, the Communists throughout Asia had quite a bit of cooperation, with many of the big players fighting in each others wars (or at least claiming to have done so, as Kim Il Sung's military career is not without controversy).

In general, though, by the 60s, China was much less involved in North Korea. Today the analysts will tell you that their continued support of North Korea (despite constantly telling them to knock off the shenanigans) is to keep the US from having a presence at their doorstep.