How did World War I change the way in which wars are fought?
Airplanes, disease and movie cameras to name three.
World War i is considered the first modern war for a number of reasons. It was the first war to use airplanes and submarines in a major way (Both had been used in other wars, but in limited capacities; a war between Italy and Albania used airplanes for reconnaissance purposes, but WWI armed airplanes for the first time.)
It used machine guns; a single soldier had the firepower to kill all the opposing combatants. (The Gatling Gun had been used in prior conflicts including the American Civil War, but with limited success.)
It was also the first conflict where more men had been killed by combat than by disease. And while some prior wars had been documented by cameras in a limited way, World War I was the first to be filmed with movie cameras.