I find it interesting that you picked the start of the war and not the events leading up to it at all.
You do realise that the US was heavily responsible for the mess that led up to it, right? Not to mention being responsible (together with the Russians) for partitioning Korea in the first place (funnily enough, the Koreans weren't even asked if this was something they'd consider desirable).
Step back a bit and you'll see that the US and USSR pretty much caused the problem. China just made it worse.
...and when you pull back further, you see nearly a half-century of foreign rule over the Korean peninsula, which ultimately led to the problem of how to reconstitute the nation, post-World War II. Shall we blame the stalemate of the Korean War on Imperial Japan - an entity that ceased to exist, five years before the war began?
Yes, reconstituting a state that ceased to exist fourty years prior is a messy business, particularly when rival parties have a vested interest in the type of state that emerges from the process. That process (which I am not defending, by the way) may have led to the war, but the outcome of the war must be attributed to the participants and how they conducted themselves, which leads me to....
Not to mention the way you consider MacArthur's actions to be a fairly minor contributor to the eventual outcome.
Not at all. MacArthur's actions were the catalyst/excuse for China's entry into the war. That makes him a pretty significant player, but China had many different ways to respond to his overstepping his authority, and they chose to enter the Korean War as a full ally of North Korea. China did not seek retribution against MacArthur; China did not seek to secure their border against his activities; China sought to join the North in wiping out South Korea. That is the decision that ultimately led to the stalemate at the Thirty-Eighth Parallel, and that's why I assign China the lion's share of the blame for that stalemate. Does MacArthur deserve a share of the blame as well, for attacking North Korean depots, within Chinese territory? Certainly. Trying to pin that part of the blame on the United States as a whole is somewhat absurd, though, since that action was taken in direct contradiction to the orders MacArthur had received from President Truman.
Oh and talking about UN resolutions at that point is kinda laughable considering neither USSR nor mainland China were involved in that vote.
1) The Soviet Union was a permenant member of the Security Council. They were not uninvolved in the vote; they gave up their vote and veto as part of a protest, regarding the representation of Communist China in the United Nations. Had the Soviet Union been so empassioned to stop United Nations' involvement in the Korean War, they could have terminated their boycott and vetoed the resolution that brought the U.N. into the conflict. Pointing out that the USSR voluntarily omitted themselves from this decision shows that they were either grossly short-sighted or placed a comparatively low priority on the outcome of the Korean War.
2) The legitimacy of The People's Republic of China was still in question, at the time the U.N. was passing resolutions relating to the Korean War. They had only managed to drive the Nationalists from mainland China months prior to the passage of UNSCR's 82 and 83, and so it was an open question of whether or not the PRC would last, as a government, long enough to warrant recognition (nevermind the Republic of China's permenant seat on the Security Council), or whether they would be toppled by a subsequent revolution or foreign invasion. (I will grant that the question should not have remained open for the two decades that it did, but with the benefit of hindsight, it's easy to see where the Chinese Civil War could have become, in the early 1950's, a proxy war on the scale of Vietnam.)
3) The point of bringing up the U.N. resolutions was to show that the United States did not go barging into the Korean peninsula unilaterally, but at the behest of the global community, and so whatever blame you would assign to the United States for the outcome of the Korean War deserves to be shared by the six other nations that voted with the United States to put U.N. support behind South Korea and the three that complacently let the resolution pass (including and especially the USSR, given their veto authority).
To find some common ground, though, I think we can both agree that Yugoslavia was blameless in this affair.
