I remember a theory published a while ago saying that the world war will happen about every 100 year. 

WWI + WWII are considered one great 30 year war. 

Napoleon war expands two decades, concluded on 1814 at Waterloo. 

Great Turkish War (1683-1699) + Nine Year's War (1688-1697) (related)

Long War (1591-1606)

Now, it is 73 year after WW2 concluded on 1945, what is next?

One theory said that the world order created after the world war cannot be maintained more than 100+ years withiout major update, due to the internal and external changes in population, politics, economy, technology advance, or environment. The one who created and maintained the order wanted to continue the good old days, while the history does not let it doing so. 

There must be something in the human history that we do not know yet.