I give you Somalia and the retreat thereof. I also give you Kosovo / The Balkans where the US army adopted their infamous Force Protection doctrine.
Not that the UN covered itself in glory in the Balkans but one expects a bit more gumption from the US Army.
The reason for that was our interests weren't involved, at least not to the extent that anyone plausibly explained. Somalia was a humanitarian operations, no one signed up for heavy casualties. Also it was Clinton still learning the ropes and probably getting bad advice. The Balkans again, no one knew what the hell we were doing there, it was already unpopular, significant US casualties and Congress would demand an immediate withdrawal. I assume you didn't really mean to question the gumption of the US army, it was purely a political decision, nothing to do with the military.