Regards amateurs, there's a report in Armor Magazine that in 1992, a Brigade that was victorious in Iraq (first Gulf War) went to the National Training Center and got their asses kicked...
Not by OPFOR, the best-trained and most experienced fighting formation in the US Army, but by a company of National Guardsmen. Weekend warriors. (To be fair, the NG boys had gotten an intensive training course from OPFOR, and the parameters of the excercise were simulating Soviet conscripts taking on experienced US Army vets.)
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