Losses were far higher than during the war with Mexico, which saw roughly 13,000 American deaths, including fewer than two thousand killed in battle, between 1846 and 1848. One reason for the high number of battle deaths in the civil war was the continued use of tactics similar to those of the Napoleonic Wars, such as charging. With the advent of more accurate rifled barrels, Minié balls, and (near the end of the war for the Union) repeating firearms such as the Spencer repeating rifle and the Henry repeating rifle, soldiers were mowed down when standing in lines in the open. This led to the adoption of trench warfare, a style of fighting that defined much of World War I.