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Mỹ (美) is the Vietnamese word for "America".
Mỹ is also the Vietnamese word for "beautiful", and it was chosen because of the respect that many Vietnamese had for the United States in the nineteenth century. Later, of course, it would become a deadly insult due to the Vietnam War.
The word entered the Vietnamese language in the mid-nineteenth century, when the shattering of Vietnam's social and political structure by the French Empire, which ruled the country from the mid-1880s until 1954, caused Vietnamese to begin looking abroad for examples of how they might reform their own society so they could become strong enough to kick out the French and regain independence. America, with its revolutionary tradition, was seen by some Vietnamese as an example worth emulating.
Because the first generation of Vietnamese to live under French rule could not read western languages, and America was on the other side of the world, early Vietnamese writers gained their impressions of what America was like from Chinese sources. In Chinese, America is Mei-kuo, and this became Mỹ in Vietnamese.
It's ironic that the Vietnamese got their word for America from the Chinese, because what Vietnamese reformers were trying to do was break their country's cultural dependence on China and lead a national revival. They naturally looked for other countries they might emulate more fruitfully. Some of the first leaders of the anti-colonial movement in Vietnam idolized George Washington for his leading role in the American Revolution and wished a figure would emerge to play a similar role in their own country. Far from being threatening, America was a country worth emulating because its technical skills and political system made it strong.
These early Vietnamese reformers failed to evict the French, and their children were educated in French-run schools across Vietnam and at universities in Paris. Here, they learned French, and in French they read Karl Marx. His writings provided a distinctly different idea of how the Vietnamese might gain independence from the French, and encouraged Vietnamese radicals to see the United States as the foremost - and hence most threatening - capitalist power in the world.
Nevertheless, Vietnamese Communists were not unduly hostile towards America at first, if only because there was no practical reason to be and because it was unwise to antagonize the world's foremost - and hence strongest - capitalist country. But when America started helping France in its war against the independence movement, and then backed the regime of Ngo Dinh Diem after 1954, Mỹ came to be seen quite differently. One of the Communists' main propaganda slogans was "Mỹ-Diem", which sought to portray the ruler of South Vietnam as an American puppet. This could be an effective slogan in a country which had fought hard for its independence.
The work of Vietnamese Communist anti-American propagandists was made significantly easier by the 1968 Mỹ Lai massacre, in which hundreds of Vietnamese civilians were shot and killed by a rogue American unit. The "Mỹ" in Mỹ Lai means "beautiful", in the original meaning of the word, but its name was now also a constant reminder of the worst things America did in Vietnam. It hadn't worked out quite as the first Vietnamese scholar who praised America the beautiful had intended.
For more on how America was viewed in Vietnam in the nineteenth century, see Mark Philip Bradley, Imagining Vietnam and America: The Making of Postcolonial Vietnam, 1919 - 1950 (Chapel Hill, NC: University of North Carolina, 2000).