My knowledge of Vietnam is pretty poor, so maybe I'm missing something obvious. However, I was reading about the history of Vietnam on Wikipedia and was puzzled by the US non-reaction to Vietnamese independence.
From my own short reading, it seems Ho Chi Minh created a government that would be amicable to the US and their interests at the time. It filled a power vacuum left by the French and Japanese, seems to have made real efforts to appeal to the US sensibilities, and seems a possibly useful ally for the approaching Cold War. From what I can see, this is before the Soviet Union was too deeply involved with them. Despite all this, it seems his letters for recognition by the United States was ignored. I've seen a couple places give the explanation that, between the death of FDR and the end of the war, the US was just too busy, but that seems a flimsy excuse to me. So, why did the US ignore this potential ally?
During WW2, the US were hardly consistent in their views about post-war Indochina, a country they knew little about. Roosevelt disliked European imperialism, and particularly French imperialism in Indochina, that he believed to be worse that that of the British in India and Malaya. He was in favour of a trusteeship over Indochina, that would consist in a neutral party exercising control over the territory for 20 or 30 years and steering patiently the Indochinese in the direction of independence. If that seems a little bit patronizing, it is: it was not that different from the future promised by French colonial theoricians like Albert Sarraut, and there is no way that Vietnamese nationalists would have accepted such a plan. The British, who had their own colonial postwar to think about, were understandably wary of Roosevelt's views on decolonization. Roosevelt did not push the trusteeship idea too hard for fear of alienating Churchill. Even if the American public opinion could be sympathetic to the idea of decolonization, Roosevelt did not have the full support of his own administration anyway. As a result, the alternative, which had US supporting French sovereignty over Indochina, was never off the table. During the Yalta Conference of February 1945, Churchill shot down the idea of trusteeship for European colonies, killing effectively the concept. De Gaulle's Free France, of course, pushed for a continued presence of France in Indochina, and promised to reform its colonial practices.
On the ground, things were a little bit different. In Spring 1944, Viet Minh representatives met Free French officials in China to discuss postwar colonial policies. They were rebuked by the French and turned to the US instead, who responded politely and voiced sympathy but told the Vietnamese that they were French citizens and that the US was committed to help France. However, after his release from a Chinese prison in September 1943, Hồ Chí Minh (HCM) had cultivated the friendship of US officers, and the name of "Mr Ho" was starting to circulate, though little was known about him. Then, in November 1944, US aviator Lieutenant Rudolph Shaw crash-landed his P-51 near the Sino-Vietnamese border. He avoided French patrols and was taken to HCM, who escorted him to China, using this opportunity to increase his status. In March 1945, HCM, in Kunming, managed to meet General Claire Lee Chenault (of the Flying Tigers fame), of the 14th Air Force. The Americans were in dire need of intelligence sources within Indochina: OSS Captain Archimedes Patti was told about this mysterious Mr Ho, and eventually met him. HCM promised Patti intelligence and assistance if his movement could be provided with weapons. Communication and trust were now established between HCM and the Americans. HCM would later tell Patti that he was willing to grant special concessions to US commercial interests in Vietnam.
In July 1945, the OSS "Deer Team" led by Major Allison Thomas was parachuted in Tan Trao, 80 km from Hanoi, where HCM, Võ Nguyên Giáp and their guerillas were now lodging. HCM convinced the Americans that he was not a "Communist Bogy" and that he stood (in Allison's words) "for freedom and reforms from French harshness". The guerrillas got weapons and training from the Deer Team. After the Japanese surrender, Viet Minh forces took over Hanoi and other cities in Vietnam. On 2 September, HCM read the Declaration of Independence to a crowd in Hanoi, quoting the American one. Americans were in attendance and two P-38 Lightning did a flyby over the crowd.
However, HCM's brief love affair with the US was coming to an end. In April 1945, a mere days before Roosevelt's death, OSS head William J. Donovan wrote a memo to the president stating that the US "should realize also its interest in the maintenance of the British, French and Dutch colonial empires." Truman, unlike his predecessor, was not opposed to the return of the French in Indochina. Still, American attitudes remained ambiguous for some time. On the ground, officers like Patti were strong supporters of HCM. In Washington, some favoured the French while others wanted a more hands-off approach. Patti could only relay to HCM the official American position: the US neither questioned French sovereignty nor supported French imperalism. HCM's letters to Truman were left unanswered. In September 1946, HCM met with US diplomats in Paris, again insisting that he was not a Communist, and hinting at the possibility of future military cooperation (the use of Cam Ranh Bay as a naval base). He reiterated his offer in December, this time in Hanoi, with no more success. HCM's "moderate" face - that of the nationalist-but-not-Communist patriot - was no longer convincing for the Americans. One should note here that while HCM later became a towering figure in international politics, he was still, in 1945, a mysterious character and not the only nationalist in Indochina. His ragtag crew of fighters had won Hanoi, but he was just one player in the chaotic Game of Thrones of 1945-1946, who involved other nationalist Vietnamese organizations, pro-French Vietnamese, French (Free and former Vichyte), British, Americans, Chinese, and, after August 1945, Japanese soldiers waiting to be repatriated and who were enlisted by the Allied or by Vietnamese nationalists.
In December 1946, Viet Minh forces attacked Hanoi, starting the Indochina war. Americans were still debating about how much of a Communist HCM really was, but the idea of helping him was no longer on the table. In January 1950, HCM turned to the newly Communist China for help, and met Mao in Beijing, where he was warmly received and obtained the official recognition of Democratic Republic of Vietnam (DRV) by China. Chinese military aid started in the following months. In February, HCM went to Moscow. Stalin was distrusting and even hostile, but, like China, the USSR recognized the DRV as the sole legal governement of Vietnam. At about the same time, Washington agreed to provide military assistance to France, and, in March 1950, Truman approved a grant of $15 million. From then, the US would only increase its support and ended up footing the bill for the Indochina War. Soviet aid would not start until the mid-1950s and became only significant after the US military escalation of the mid-1960s.
In a nutshell, the US never really imagined what a postwar Indochina could be. The only plan formulated before the end of WW2 was a trusteeship that went nowhere. HCM was great at charming the hell out of individuals (and some US officers were really in love with the guy), but his personal charisma could not counteract the fact that he was a newcomer and an unkown quantity in a chaotic scene, and a potential Communist, when the returning French had De Gaulle, Leclerc, De Lattre and other experienced leaders, a colonial administration that could be restarted, and the support of many Vietnamese who had no reason to trust HCM.
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