HO CHI MINH CITY, Vietnam — An American who has been a resident here for a few years said to me the other day: “You know, they still look at us here the way we want to be looked at. America equals opportunity, entrepreneurship and success. That’s not true in so many places anymore.”

Four decades after the war, in one of the world’s consoling mysteries, the United States enjoys an overwhelming approval rating in Vietnam, reflected in the outpouring of enthusiasm for President Obama during his three-day visit last month. In this fast-growing country of 94 million people, about one-third of them on Facebook, America is at once the counterbalance to the age-old enemy, China, and an emblem of the prosperity young people seek.

The best way to kick Vietnamese aspirations in the teeth, turn the country sour on the United States, and undermine the stabilizing American role in Asia, would be for Congress to fail to ratify the Trans-Pacific Partnership, Obama’s signature trade agreement with 11 Pacific Rim countries including Vietnam but not China.

If T.P.P. falls apart, China wins. It’s as simple as that. Nonratification would signal that Beijing gets to dictate policy in the region, and the attempt to integrate Vietnam comprehensively in a rules-based international economy fails.