Outside of the retreat in Korea after the entry of China I can't think of an example where the US lost a tactical encounter after WW2. We lost wars but not due to military reasons. We were not defeated on the battlefield in Vietnam or Afghanistan.
When at first you don't succeed, redefine success?
And for the people talking about Vietnam.. a lot has changed in 50 years. America got stronger while most other nations, except China, got much weaker. China is the single plausible refutation to my claim above.