World War II significantly outranks other wars throughout history in several ways. Geographically the war involved more countries, was fought on more fronts and involved battles in more major oceans than any other war in history. Casualties count over 50 million service members and civilians making it the deadliest war ever fought.
Socially and politically, the war literally changed the world because it involved so many countries. The United States came into its own as a major power and ended the war as a super power with Russia. European countries lost political strength.
Technology made enormous strides as the war effort made demands on technology developers. Communication devices advanced as did weaponry and vehicles. Women joined the workforce in significant ways for the first time, as did African Americans, changing the face of businesses. This influx of activity into businesses also helped end the Great Depression in the U.S.
Lastly, the creation and use of the first Atomic Bombs ushered in a new era of terrifying war time capabilities that had the potential to cause massive casualties and devastation.