My understanding of Swiss neutrality (simplistic and very poor) is that the swiss mercenaries were too good and after they were finally defeated a ban on fielding mercenary companies was enforced and this lead naturally to the neutral stance Switzerland takes today.

https://en.wikipedia.org/wiki/Swiss_mercenaries

Except the pope, being the pope got to keep his.

It's not a reputation but an actual policy. It's hard to untangle but the policy has more to being surrounded by other states (no ocean) and having big walls around it already, and being too small for geopolitical strength.

The country was last invaded in the napoleonic wars.