And I'm sure the US thought that the technologically poor North Vietnamese would fall quickly too. You can't underestimate the role of the people's ideological identity when it comes to taking over a country.
Unlike in Vietnam, I suspect that the Chinese leadership will be committed to a long term conflict.
America never had national identity at stake in Vietnam. China has claimed Taiwan since Mao Ze Dong. Taiwan is a way bigger deal for the Chinese national identity than Vietnam ever was for the US.
More importantly the Chinese leadership doesn’t need to worry about an election cycle. They can easily think in 20-40 year timeframes, while American leaders can go 4 years ahead at most.
