The harsh environment, constant intertribal warfare and the blending of horsemanship, survival skills and archery made the Mongols (probably) the most terrifying force ever seen in human history. It can be noted that because they grew up with war and struggle for survival as a constant everyday things, this qualifies as the best possible training system. It cannot be intentionally implemented. There is a lot of evidence in favor of this throughout human history. It was always the nomadic tribes of Central Asia that overthrew the greatest of empires and cultures.
As far as intentional training, the Romans probably proved theirs to be vastly superior on the battlefield better than any other.
"It is well that war is so terrible, else we should grow too fond of it."
- General Robert E. Lee
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"It takes a big man to admit when he's wrong. I am NOT a big man." -Chevy Chase