I'm sorry but I can't let gun cliches go by. If we argue from cliches we lose credibility.
We have shooters who have thought about armed response but we have clear ones who don't. The samples and unknown cases make it hard to quantify deterrence, esp. against those who are sucidial - 40% at least. We can't know who was deterred but we certainly know those who weren't.
Second, the Switzerland and Japanese examples are more complex than a gun cliche.
1. The Japanese had no known plans to invade the USA mainland. The supposed source cannot be proven to be such. The Japanese plans are well known and I've read some scholarly texts on such. The threat was never real or even planned. To quote a suspect quote makes us look less than credible.
2. Switzerland - Switzerland did plan for resistance but the real reason for no invasion were many fold:
a. The Swiss collaborated with the Nazis - it is very well known. They gave Hitler most everything he wanted. Thus the German felt no need to invade. They did have invasion plans if needed and they would have conquered them with a bit of trouble. It would be a diversion and not worth it since they got everything they wanted.
b. The real military threat was the destruction of Swiss tunnels to Italy that would negatively impact German forces in Italy. That was more important than Swiss armed resistance in their calculations.
3. Columbine was supposed to be a sophisticated bombing. The shootings happened when the bombs failed. Armed teachers don't deter bombers.
Antigunners are prone to hyperbole. We shouldn't be.