What french fortress city under the command of Henri Petain help off a massive German assault?

The French fortress city that held off a massive German assault under the command of Henri Pétain was Verdun. The Battle of Verdun, fought from February to December 1916 during World War I, became one of the longest and most grueling battles in history. Pétain famously declared, "They shall not pass," highlighting the determination to defend the city against the German offensive. The battle resulted in heavy casualties on both sides, but Verdun remained under French control.