Even today, any modern military will never underestimate the effect of "boots on the ground."  Perhaps the element of warfare that has changed the most throughout history, infantry still play an important part of any army. Many battles were fought on open battlefields between rival soldiers and armies, however the introduction of cannons, artillery, and later vehicles such as aircraft and tanks, have changed how wars are fought. World War I saw lots of trench warfare, while World War II brought the concept of mechanised infantry, with soldiers and vehicles working in unison, to the forefront. Guerilla warfare became much more prominent in the 20th Century, particularly in occupied countries. The role of the modern soldier is constantly changing.