This is probably my longest post ever written on r/AH.

There isn't a simple answer to this question (I mean, there hardly are ever simple questions in history) because no one, or at least no documentation exists to my knowledge of, someone in a Western position of authority saying, "We can't ally and arm Tibet to force China into a multi-front war because X."

That said when looking at the whole picture, we can see how Tibet was very easily overlooked, both for Tibetan and Allied reasons.

In short: the United States was in newfound astronomical heights as a superpower, replacing the old order of the British Empire, which was (essentially) the only Colonial power that had any relations at all with Tibet. India had inherited those relations, but Nehru was eager not to play Cold War politics (it would not go well, as we shall see), and between Truman's containment policy, British decolonization, and American ignorance of the situation in the Far East, Tibetan isolation made being ignored as a reliable anti-Communist partner an all but inevitability.

I. American Empire in the Pacific

Following World War II, the United States found itself in an unprecedented position of hegemony in the Pacific Ocean. Not only was Hawai'i secure, but the Philippines were fortified (MacArthur had followed through with his promise), Australia and New Zealand were strong, stable allies, and the Japanese Empire was, essentially, conquered. Between bases leased to the US by the UK (I'll use these abbreviations, though when I type "UK," "British Empire" would probably be more appropriate in historical terms), occupation zones, and annexations from the Empire, the United States had basically total control.

The US had an interesting cultural perspective of China: they viewed it as "their" Africa. China was the ultimate in US Manifest Destiny: it would be the inevitable geopolitical accomplishment of the Columbian Exchange. There were millions of souls to be ministered to, millions of customers just ripe for open American markets, millions of laborers that could be imported paid cheaply to do things like build railroads and work sugar and pineapple plantations. This obviously very colonialist viewpoint came crashing down with the Red Scare and the victory of the Communists in the Chinese Civil War.

Chinese and Chinese-Americans, who once came under suspicion by Americans mistaking them for Japanese during the war, now came under actual suspicion by American law enforcement, many of them being surveiled and kept under watch - especially regarding any political activity. There was very much a sense of cultural betrayal by the Chinese throwing in their lots with the Reds. The US, seemed to have an endless amount of love for anyone and anything that was anti-Communist, and the flight of the Kouimintang (KMT) to Taiwan was the beginning (but not necessarily the prologue) of the US' problematic relationship with Asian dictatorships.

Chinese and Chinese Americans once proud (or at least safe) to advertise their Chinese-ness during the war against Japan, now found that those same identity markers put targets on their backs. But high KMT officials, Chiang Kai-Shek, Soong Mei-ling (his wife), and Anna Chennault (originally Chan Sheng Mai), all had high profile if not state-level visits with US Presidents (not where we are, but Google Nixon, North Vietnam, and Anna Chaunnault... it's... mind blowing).

While Red vs. Not-Red was easy enough for Americans to grasp, and the cultural shift from China-Our-Colonial-Destiny to Red Menace was pretty shocking, but at least comprehensible to most Americans. And dare I say, that the racial understandings of most Americans was significantly lower than it is today. Hell, read a few books about Korean culture in the West and most Koreans seem happy to be just identified as Korean... in the early 2000s.

That's all to say that beyond Chinese vs. Japanese, Reds vs. ahem Whites, Americans, even higher up officials like the President and Secretary of State weren't prepared to really think about the complexities of Asian geopolitics. I use the term Racism [1] not in a malicious way, but just as a reference to how at least the situation in Europe could be grasped even if to say the grievances of the Serbs against the Germans maybe wasn't "understood" in any deep sense of the term. Let's remember that a few decades earlier, the US annexed Puerto Rico, Guam, and the Philippines, and refused to extend full Statehood to any of these places, with the explanation being that the "alien races" that inhabited these zones could not understand "Anglo-Saxon Principles." I think it's safe to say that the United States saw Latinos, Pacific Islanders, and Asians as "lesser" and to varying degrees as "uncivilized."

This is relevant because these kinds of presuppositions and starting points affect decision makers and the way they interact and perceive the people, cultures, and ultimately, politics of others. [2]

For example, Christian missionaries had been swarming the Korean Peninsula all throughout the 1800s. Pyeongyang was even known as the "Jerusalem of the East" it had become so Christian. Yet when MacArthur made his victory lap after the initial reconquest of Seoul, he started off his speech by saying, "Now I know this isn't a Christian Nation, but I'd like to offer Thanks in the form of the Lord's Prayer." The man for whom he had prosecuted the whole war in favor of, Syngman Rhee, was a Methodist Christian and was raised in Hawai'i. He would have been well familiar with the Lord's Prayer. (Side note, the Kim family on the other side of the front lines were also descended from Christian Missionaries, and would have known the Prayer by heart as well, as I'm sure Billy Graham could have attested to.)