An interesting natural experiment happened on the Pacific Theater of WWII. American and Canadian forces attacked an island which had been secretly abandoned by the Japanese weeks prior. Their unopposed landing resulted in dozens of casualties from friendly fire and dozens of men lost in the jungle. Presumably, a similar rate of attrition occurred in every other landing, on top of casualties inflicted by the deliberate efforts of enemy troops.
If the landing had been peaceful & uneventful, perhaps we wouldn't have heard about it. So there might be a selection effect.
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