Final answer:
Napoleon's greatest failure was the Invasion of Russia in 1812, which led to devastating consequences for his army and reputation.
Explanation:
Napoleon's biggest failure was the Invasion of Russia. In June 1812, Napoleon led a massive army into Russia with hopes of a quick victory but faced challenges like the tactical retreat of the Russians, burning of crops, severe winter conditions, lack of supplies, and diseases. This invasion resulted in a significant loss of his forces and questioned his military prowess.
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