World War I was considered a total war because all of the population of the main nations involved were called into service in one way or another. Even women, who typically were not involved in war, were responsible for growing food supplies and working in artillery factories.
The term "total war" was not created until the 1930s by a German general, which was after World War I, but preceding World War II. Both of these wars put incredible strain on all aspects of the nations involved. Governments passed laws that would normally never be acceptable in order to bolster wartime efforts.