Switzerland hasn't been invited for 800 years precisely because it does have a considerable army (to wit: every able-bodied male), not for want of would-be conquerors.
A famous exchange between German and Swiss military leaders:
"We have 400,000 troops ready to invade. You have but 200,000 civilian militia. What will your people do when ours arrive?"
Also, the terrain is notoriously difficult, the bridges have predetermined break points and places to put the dynamite (none there atm, but not so long ago there was), and any old shed in the mountain could contain a bunker or hangar. Apart from Israel, Switzerland is the only country with enough bunkers for the whole population (even ABC rated I think).
Then how did the Helvetic Republic come about? During the wars of the French revolution it had pitched battles fought on its territory by outside forces (some of which had Swiss regiments on payroll, if not on the battlefield). Foreign powers moved large military forces through Swiss territory for many centuries.
I stand corrected. For 5 years, France attempted to instill a puppet government over the otherwise loosely aligned Swiss cantons. The attempt failed 211 years ago.
A famous exchange between German and Swiss military leaders:
"We have 400,000 troops ready to invade. You have but 200,000 civilian militia. What will your people do when ours arrive?"
"Shoot twice and go home."
No invasion.