What's the most self-defeating strategy (in terms of military posturing and administration rather than morality/ideology) that each of the countries participating in World War Two could have attempted, but didn't? However, it should still be something that they could have plausibly tried, even if it was doomed to failure, it must be something they would actually consider doing.
For example, Operation Pike would have been a terrible idea for Britain, luckily that idea was scrapped.