The great advantage of having nuclear weapons is that large scale wars became impossible - WW III and IV were not fought. Had the Communist world attacked the West (or vice versa) the world would have suffered greatly - the destruction of Europe and democracy/capitalism might have resulted.

Greatest lost to humanity? The death at 2 months of the worlds greatest genius, an unknown child who never lived long enough in the last 500 years to delivery up utopia.


More realistically: The decision of China to enter into isolationism instead of continuing its expansion in the 15th century.