In Quillette, Niranjan Shankar outlines the world situation that led to the outbreak of the Korean War in 1950:

Initial phase of the Korean War, 25 June through 5 August, 1950.
Map from the West Point Military Atlas — https://www.westpoint.edu/academics/academic-departments/history/korean-war

The Korean War was among the deadliest of the Cold War’s battlegrounds. Yet despite yielding millions of civilian deaths, over 40,000 US casualties, and destruction that left scars which persist on the peninsula today, the conflict has never received the attention (aside from being featured in the sitcom M*A*S*H) devoted to World War II, Vietnam, and other 20th-century clashes.

But like other neglected Cold War front-lines, the “Forgotten War” has fallen victim to several politicized and one-sided “anti-imperialist” narratives that focus almost exclusively on the atrocities of the United States and its allies. The most recent example of this tendency was a Jacobin column by James Greig, who omits the brutal conduct of North Korean and Chinese forces, misrepresents the underlying cause of the war, justifies North Korea’s belligerence as an “anti-colonial” enterprise, and even praises the regime’s “revolutionary” initiatives. Greig’s article was preceded by several others, which also framed the war as an instance of US imperialism and North Korea’s anti-Americanism as a rational response to Washington’s prosecution of the war. Left-wing foreign-policy thinker Daniel Bessner also alluded to the Korean War as one of many “American-led fiascos” in his essay for Harper’s magazine earlier this summer. Even (somewhat) more balanced assessments of the war, such as those by Owen Miller, tend to overemphasize American and South Korean transgressions, and don’t do justice to the long-term consequences of Washington’s decision to send troops to the peninsula in the summer of 1950. By giving short shrift to — or simply failing to mention — the communist powers’ leading role in instigating the conflict, and the violence and suffering they unleashed throughout it, these depictions of the Korean tragedy distort its legacy and do a disservice to the millions who suffered, and continue to suffer, under the North Korean regime.

Determining “who started” a military confrontation, especially an “internal” conflict that became entangled in great-power politics, can be a herculean task. Nevertheless, post-revisionist scholarship (such as John Lewis Gaddis’s The Cold War: A New History) that draws upon Soviet archives declassified in 1991 has made it clear that the communist leaders, principally Joseph Stalin and North Korean leader Kim Il-Sung, were primarily to blame for the outbreak of the war.

After Korea, a Japanese imperial holding, was jointly occupied by the United States and the Soviet Union in 1945, Washington and Moscow agreed to divide the peninsula at the 38th parallel. In the North, the Soviets worked with the Korean communist and former Red Army officer Kim Il-Sung to form a provisional “People’s Committee”, while the Americans turned to the well-known Korean nationalist and independence activist Syngman Rhee to establish a military government in the South. Neither the US nor the USSR intended the division to be permanent, and until 1947, both experimented with proposals for a united Korean government under an international trusteeship. But Kim and Rhee’s mutual rejection of any plan that didn’t leave the entire peninsula under their control hindered these efforts. When Rhee declared the Republic of Korea (ROK) in 1948, and Kim declared the Democratic People’s Republic of Korea (DPRK) later that year, the division was cemented. Each nation threatened to invade the other and began preparing to do so.

What initially prevented a full-scale attack by either side was Washington’s and Moscow’s refusal to provide their respective partners with support for the military reunification of the peninsula. Both superpowers had withdrawn their troops by 1949 to avoid being dragged into an unnecessary war, and the Americans deliberately withheld weapons from the ROK that could be used to launch an invasion.

However, Stalin began to have other ideas. Emboldened by Mao Zedong’s victory in the Chinese Civil War and frustrated by strategic setbacks in Europe, the Soviet premier saw an opportunity to open a “second-front” for communist expansion in East Asia with Beijing’s help. Convinced that Washington was unlikely to respond, Stalin gave Kim Il-Sung his long-sought “green-light” to reunify the Korean peninsula under communist rule in April 1950, provided that Mao agreed to support the operation. After Mao convinced his advisers (despite some initial difficulty) of the need to back their Korean counterparts, Red Army military advisers began working extensively with the Korean People’s Army (KPA) to prepare for an attack on the South. When Kim’s forces invaded on June 25th, 1950, the US and the international community were caught completely off-guard.

Commentators like Greig, who contest the communists’ culpability in starting the war, often rely on the work of revisionist historian Bruce Cumings, who highlights the perpetual state of conflict between the two Korean states before 1950. It is certainly true that there were several border skirmishes over the 38th parallel after the Soviet and American occupation governments were established in 1945. But this in no way absolves Kim and his foreign patrons for their role in unleashing an all-out assault on the South. Firstly, despite Rhee’s threats and aggressive posturing, the North clearly had the upper hand militarily, and was much better positioned than the South to launch an invasion. Whereas Washington stripped Rhee’s forces of much of their offensive capabilities, Moscow was more than happy to arm its Korean partners with heavy tanks, artillery, and aircraft. Many KPA soldiers also had prior military experience from fighting alongside the Chinese communists during the Chinese Civil War.

Moreover, as scholar William Stueck eloquently maintains, the “civil” aspect of the Korean War fails to obviate the conflict’s underlying international dimensions. Of course, Rhee’s and Kim’s stubborn desire to see the country fully “liberated” thwarted numerous efforts to establish a unified Korean government, and played a role in prolonging the war after it started. It is unlikely that Stalin would have agreed to support Pyongyang’s campaign to reunify Korea had it not been for Kim’s persistent requests and repeated assurances that the war would be won quickly. Nevertheless, the extensive economic and military assistance provided to the North Koreans by the Soviets and Chinese (the latter of which later entered the war directly), the subsequent expansion of Sino-Soviet cooperation, the Stalinist nature of the regime in Pyongyang, Kim’s role in both the CCP and the Red Army, and the close relationship between the Chinese and Korean communists all strongly suggest that without the blessing of his ideological inspirators and military supporters, Kim could not have embarked on his crusade to “liberate” the South.

Likewise, Rhee’s education in the US and desire to emulate the American capitalist model in Korea were important international components of the conflict. More to the point, all the participants saw the war as a confrontation between communism and its opponents worldwide, which led to the intensification of the Cold War in other theaters as well. The broader, global context of the buildup to the war, along with the UN’s authorization for military action, legitimized America’s intervention as a struggle against international communist expansionism, rather than an unwelcome intrusion into a civil dispute among Koreans.