World War 1 was fought in Europe, the Pacific, and Africa. It involved cavalry, trench warfare, and mechanized combat. While World War I is known primarily for trench warfare, it also involved mobile combat and the movement of tens of thousands of men, machines, and animals. There were numerous, bloody days and battles in World War I, but the most deadly took place during the opening days of the conflict, at the Battle of Frontiers, where 27,000 French soldiers were killed in a single day.