Charlemagne crossed the Alps during his 773 invasion of Lombard Italy.
The mountain crossing was the opening move in a campaign undertaken after Lombard king Desiderius threatened papal territories and rejected Charlemagne’s diplomatic demands. Charlemagne and his uncle Bernard led the Frankish army south through the Alpine passes into northern Italy, where Lombard fortifications initially blocked the route.
Rather than abandon the invasion, Charlemagne’s forces outflanked the defenses in the Susa Valley. The Lombards retreated toward Pavia, their capital, and Charlemagne began a siege that lasted into 774. Pavia’s fall ended Desiderius’s reign and allowed Charlemagne to adopt the title King of the Lombards.
The Pyrenees are a tempting alternative because Charlemagne later campaigned in the Iberian frontier, including the expedition associated with the Battle of Roncevaux in 778. The Apennines run through Italy but were not the major mountain range identified with his entry into Lombard Italy in 773; the defining crossing was the Alps.