If the USA is such a powerful nation, why couldn't it defeat Vietnam and Afghanistan?

What great military nation has the USA ever beat on its own? Russia beat the Mongols, the Nazis, and Napoleon. The UK, Spanish, and French conquered the world, defeating indigenous people around the world. The precursor to Germany beat the Roman empire and the invading Muslim army of tours. What's so great... show more What great military nation has the USA ever beat on its own? Russia beat the Mongols, the Nazis, and Napoleon. The UK, Spanish, and French conquered the world, defeating indigenous people around the world. The precursor to Germany beat the Roman empire and the invading Muslim army of tours. What's so great about the USA? And besides how much it spends on its military, why is it regarded as a powerful nation?
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