From a review by Dave Kopel:
...although the Germans several times massed troops on the Swiss border for an invasion, the invasion never went forward. With so many reasons to invade Switzerland, why did the Nazis desist?
The Nazis could have eventually have conquered Switzerland, but at a fearful price. The Wehrmacht expected 200,000 German casualties; it would have taken a very long time to remove the Swiss military from the Alpine “Reduit” to which they planned to make a stand. And by the time the Swiss were defeated, every bridge and train track and everything else of value to the conquerors would have been destroyed.
The reason that Switzerland was too difficult to invade—in contrast to all the other nations which Hitler conquered in a matter of weeks—was the Swiss militia system. Unlike all the other nations of Europe, which relied on a standing army, Switzerland was (and still is) defended by a universal militia. Every man was trained in war, had his rifle at home, was encouraged to practice frequently, and could be mobilized almost instantly. The Swiss militiaman was under orders to fight to the last bullet, and after that, with his bayonet, and after that, with his bare hands. Rather than having to defeat an army, Hitler would have had to defeat a whole people.
It's an interesting piece of history you don't hear much about. The book can be a little dry & repetitive at times, but other times it is fascinating.
Mr. Halbrook wrote a second book on the subject: The Swiss & the Nazis: How the Alpine Republic Survived in the Shadow of the Third Reich. Haven't had the chance to read it yet.