Why invading Switzerland in 1939 is a poor German alternative

What if Germany invaded Switzerland instead of Poland in September 1939?

Germany would face rapid Swiss mobilization, hostile terrain, worsening weather, and likely intervention by France, Britain, and Italy.

German prospects in Switzerland were poor even before considering foreign intervention. The Swiss armed forces could mobilize quickly and draw on weapons, caves, bunkers, and prepared mountain positions. A September campaign would also race the weather before Alpine valleys and passes became even harder to use. France and Britain would probably intervene, and Italy could oppose a German move that threatened the Alpine balance.