Hard to tell, we can only speculate
Facts, unternehmen Tannenbaum exist, Hitler wanted to rule over All Germans under one Reich, Mussolini wanted the Italian part of Switzerland for his new roman empire.
Why was unternehmen Tannenbaum never realized ?
it was part of Invasion plans for France were wehrmacht opens a second front at French south east border to get to Rhone valley.
but fast surrender of France made this unnecessary and Hitler and Wehrmacht had other focus on Britain and USSR
next to that Switzerland sign a treaty with Third Reich and Italy, providing free transport over Gotthard train line and be supplier for German War industry.
This makes unternehmen Tannenbaum total unneeded
But what if ?
Switzerland refused to sign the Treaty and south France refuse to surrender
Hitler would order unternehmen Tannenbaum.
but a fast and easy victory it will be not, not on long term
The Wehrmacht will manage on flat land of Switzerland, while the Italian Army is unable to perform...
But the Swiss Army is prepared and will move toward the Mountains were German heavy equipment is ineffective
here the Swiss Army can rely on series of Bunkers and Fortress build into Mountains
from here they do guerrilla war fare against Germans and Italians
I guess they could manage there guerrilla warfare until 1945.
it would interesting if after the War, Switzerland makes claim on German and Italian territories as compensation for the Invasion