From Wikipedia:
The Battle
of Marignano was fought during the phase of the Italian Wars
(1494–1559) called the War of the League of Cambrai; and took
place on 13 and 14 September 1515, near the town today called
Melegnano, 16 km southeast of Milan. It resulted in a victory
for French forces. It was one of the most savage and (for the
Swiss) decisive battles of the age.
It pitted on the one side the French army, composed of the best
armored lancers and artillery in Europe; and led by Francis I,
newly crowned king of France and a day past his 21st birthday.
On the other the heretofore invincible Swiss Mercenaries of the
Old Swiss Confederacy, considered the best infantry in the
world. With Francis were some German landsknechts, bitter rivals
of the Swiss for fame and renown in war; and arriving late his
Venetian allies.
The campaign of Marignano followed years of Swis successes;
during which French fortunes in Northern Italy had suffered
greatly. The Swiss had taken control of Milan (for France the
gateway to Italy) after their victory at the Battle of Novara
(1513), and returned to its ducal throne Massimiliano — son of
Lodovico il Moro, the last duke of the House of Sforza to rule
independent Milan; as their puppet.
The prologue to the battle was a remarkable Alpine passage, in
which François hauled pieces of artillery (including 40 or 70
huge cannons) over new-made roads over the Col d'Argentière, a
previously unknown route. This was, at the time, considered one
of the foremost military exploits of the age; the equal of
Hannibal's crossing of the Alps.