Since AP is pushing hypothetical wars:
How about the Invasion of Switzerland? I passed through Switzerland a few years ago (chaperone for my daughter's high school graduation trip). The countryside is spectacular but most definitely NOT flat. I cannot see anybody really managing to conquer, let alone pacify this mountainous country. It would be interesting to see the vaunted Wehrmacht make the attempt. Of course, I do not really wish any such calamity on anybody.
2,500 years ago people worshiped cats. The cats have never forgotten this!