Reaching Grenonle was the key objective for the Germans on the Alpine Front, because there they could link up with their new Italian allies (who had declared war on France on June 21). colonel Von Funk decided to take the city via a night time surprise attack, but the phone lines were still working and the French were able to get word of the attack to their forces in Voreppe (a town in the narrow Isere Valley). Infantry, engineers, Marine gunners, a few 75mm guns, some 47mm antitank guns and two huge 194mm tracked self-propelled guns were deployed along the valley on the morning of June 23, several of its tanks were quickly destroyed. A second frontal assault achieved similar results, so the attackers fell back to prepare a more careful battle plan.

The Germans gathered all the artillery they could and began bombarding French positions late in the afternoon. They then sent in an infantry assault but it was stopped by the French artillery, which benefited from excellent spotting by officers positioned on the nearby mountain tops of the Vercors plateau. Several German tanks opened fire on French positions at long range, but French anti-tank fire responded and killed two more German tanks. The Germans abandoned all hope of taking Voreppe with a frontal assault, and planned to envelop the French the next day by moving through the mountains to the east. but by then the French had brought in even more artillery, and all further German efforts met with similar fate.