World War III
When the two largest Wars in the history of the world have been named incrementally, it is impossible not to wonder if there will ever be a third to the list.
Of course, no two wars are fought the same. The first world war was the last of its kind because the advent of viable aircraft changed everything. Today, I believe that nuclear weapons would have a significant impact. If not used outright, the threat of their use would still hang over the battlefield and change the way decisions were made.
I also think whether or not a massive conflict of World War scale is inevitable. History has shown that civilizations have risen, ruled, and inevitably fallen. America is also a civilization, although still very young. It is remarkable how quickly this country has become what it is, but I think that the fresh slate that these lands offered only a few hundred years ago has proved to be a tool for unbridled development rather than a handicap.