In my opinion, World War II was the last "world war."
Quite unlike what were the cases in World War I and World War II -- with the former being mostly a major conflict between the "allies" and the Germans and the latter being mostly a major conflict between the "allies" and Germany-Japan -- any global conflict in the future will be the "allies" against a whole bunch of "masked men."
In other words, I believe that what will happen in the future is the "allies" will go to war against largely nondescipt factions.
And opposite of what was case in both World War I and World War II, there will not be any declarations of war -- and neither side will ever be beaten into submission and go on to sign a truce or formal statement of surrender on a U.S. Navy warship.
Furthermore, the conflict will not merely consist of the opposing sides shooting back and forth and carrying out various kinds of military-guerilla-terrorist attacks against each other's people, cities and lands.
Much of it will consist of such things as knocking out nationwide communications systems, contaminating food and water supplies, and releasing and spreading the viruses of deadly and highly-contageous diseases.
In summation, when World War III starts and gets underway, no one will even know that such a conflict is taking place, because it will appear to be little more than a stepped-up version of what is already happening today -- which brings something to mind.
Perhaps World War III has already begun. Maybe all of the terrorism, unrest, and cyber attacks against national security agencies and large private organizations, are the goings-on of the early days of World War III.