France wanted to maintain its empire. This is costly and expensive, so they wanted help to do this. They used the risk of them becoming neutral in the cold war to push the US into supporting them in Vietnam. They didn’t use the NATO alliance to force assistance, this wasn’t clear in my comment. NATO country’s like the UK didn’t get involved in Vietnam, even with the US offering trying to pay them to send troops (UK was uniquely adapt at Jungle fighting relative to other western powers).

It may not seem like it today but early cold war post world war 2 the west wasn’t as unified as it is today. British and France seriously consider themselves being a third way, continuing their imperial past. Not communist like Russia or Liberal like the US. Eventually the US through influence and might pushed them into fully liberal countries. America’s liberal constitution/history made it’s politicians and political culture anti imperialist. But they were far more anti-fascist and anti-communist, so they accepted western Europe as allies.

America’s anti-communist actions often had it labelled as the anti-imperialist imperialist. But their ‘empire’ usually consisted of getting locals to get the country running again with US military and finical backing. This resulted in the US becoming strong allies with their conquered countries despite those countries having completely independent control of their nation and foreign policy. Like Japan and Korea (it could have been possible with Afghanistan if the Taliban didn’t immediately take over).