[michaelt]

There were fears at the height of the cold war that the US wouldn't launch their nuclear weapons if soviet tanks literally entered Paris because of the resulting mutually assured destruction. This lead to the formation of ideas like the tripwire force [1]. If you're interested in such topics, I can recommend "The Strategy of Conflict" by Thomas C. Schelling.

If there were credibility problems then, when America was on a much more martial and pro-free-europe footing, the situation is hardly any better now. Although there is a lot less fear of Russian tanks entering Paris, certainly!

[1] https://en.wikipedia.org/wiki/Tripwire_force