Switzerland became a neutral country after it lost the Battle of Marignano in 1515
After they lost the Battle of Marignano in 1515, the Swiss decided to become a neutral country.
The battle was fought between France and led by their newly crowned, 21-year-old king, King Francis I, and the Swiss Confederacy.
France did have help in the form of German landsknechts and also from its Venetian allies.
Lengthy negotiations followed and on November 29, 1516, the two countries signed an ‘Eternal Peace’ in which both vowed that they will seek diplomatic or judicial resolution of all future conflicts.
Switzerland has been neutral for almost 500 years, which means it will not participate in any war between other states. It will, in other words, not choose sides and will not assist any side against another.
Neutrality has not only protected Switzerland from war, but has also protected the country from being torn apart when its own different language communities might have been tempted to side with different countries in times of conflict.
The Swiss have been following the advice of their own St. Nicholas of Flüe who said "Don't get involved in other people's affairs," but in October 2002 the first armed Swiss peacekeepers arrived in Kosovo.
Switzerland stressed that it was motivated by the desire to promote peace and security and that it reserves the right to withdraw if it believed its neutrality was threatened.