Leonardo,
I have to congratulate gtho4 for his excellent answer. My research indicates that he is correct. I found the same reference that he quoted in the Swiss History section “From the Swiss Confederation to the Constitution of 1848" in the Encyclopedia Britannica. This battle was fought during the Valois-Habsburg Wars.
I also checked all European wars subsequent to 1515, namely, The Thirty Years’ War (1620 - 1648), the Napoleonic Wars (1797 - 1815), the Austro-Prussian War of 1866, the Franco-Prussian War (1870-1871), WWI and WWII. I also noted the invasions as mentioned by gtho4, but none involved a foreign battle. The Swiss maintained neutrality throughout.
Also:
“Switzerland
“In 1515, the French defeated the Swiss at Marignano in Italy. The Swiss suffered great losses, and began to question their policy of expansion. Switzerland soon adopted a policy of neutrality, and has stayed out of foreign wars ever since.”
World Book Millenium 2000
So the references agree: The Swiss Army fought its last foreign battle at Marignano in northern Italy on September 13, 1515.
Again, great job gtho4!
Leonardo,
I prepared the above off-line and just now see that the answer is too late for you.
Sorry to have taken so long, but I've been out each of the last three evenings and not had time to work on this forum until now.
Try us again with another "impossible" question -- maybe we'll answer it in time.
Cordially,
Russ