Biggest military upset ever?

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First Italo-Ethiopian war

I’d say that, no wonder that Ethiopia was one of the rare African countries to resist colonization in the 19th century

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Pearl Harbor as the Japanese’s main target were American aircraft carriers that weren’t even there at the time of the attack and these aircraft carriers were crucial for the US to win the Battle of Midway

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Another upset would be Russia’s 2022 offensive on Kyiv. Many people thought that Russia would take Kyiv within something like 2 weeks but Zelenskyy stayed there and because of the courage and resilience of Ukrainian troops and also because of Russia’s bad tactics and unfavorable meteorological conditions (muddy ground and weather), they have derailed Russia’s attempts to take Kyiv thus giving hope for Ukraine