The medieval event that involved a conflict between English and French monarchs is the Hundred Years' War, which lasted from 1337 to 1453. This protracted series of battles and skirmishes was primarily fought over territorial disputes and claims to the French throne. The war saw major battles like Crécy and Agincourt, alongside significant developments in military technology and strategies. It also fostered national identity and unity in both England and France. Ultimately, the war concluded with the expulsion of English forces from most of France, solidifying the power of the French monarchy and significantly shaping European political landscapes.