Back when the Wehrmacht was pouring concrete all over the whole northern coast of Europe like it was an olympic event, in order to create the Fortress Europa, does anyone think they were doing it because they were planning to just meekly give up and turn it all over to the allies when they showed up? Fortifying your position is what you do when you have every intention of out-staying your welcome, and want everybody to know it.
It's also what you do when you have reason to believe there is credible threat of an imminent attack.
Even if you're going to have to do that yourself too.
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