This video is a bit less than 20 minutes long. It has been making the rounds on Facebook and other social network sites, so it is possible you have seen it already. If you have not, you should. It is incredible.
Numbers surrounding the Second World War are always ripe for debate, of course, and if you view the comment thread on Vimeo you will see that the debates have already started. The only revision I would make to the video does not concern the Second World War at all, but the An-Lushan Rebellion (755-753) fought a thousand years before it. This rebellion is often included in lists of the world's deadliest wars and it shows up when Mr. Halloran compares the Second World War's death toll to that of earlier conflicts of equal consequence.
While it was surely a destructive event, I do not think there is proper evidence to prove that it was that destructive. The 36 million casualties number comes from Tang Dynasty censuses that showed the population of China just before and just after the rebellion, with 36 million being the difference. Many of those 36 million people surely died in the rebellion, but many more fled and moved to safer, more remote locales. The number should be properly understood not as the number of civilians killed, but as a measure for how badly the Tang government's ability to monitor and control the Chinese population it governed had been damaged. It was a war from which the Tang would never recover.
In this sense, it was a very different kind of conflict than the Second World War, a war whose legacy is now seen mostly in the realm of memory. The An-Lushan Rebellion was (from a Chinese perspective) a war that ravaged the known world and involved almost all of the important military powers of its day. While bright emperors like Xianzong (r. 805-820) would try to pull the Tang back together in the decades after the rebellion, the dynasty's decline was terminal. The forces unleashed by the war eventually led to the complete disintegration of Tang power. This kind of collapse was not seen after the Second World War. The power that suffered the most was to emerge from the conflict as the world's second strongest. But it was not just the Soviet Union that showed remarkable resilience--humanity as a whole weathered the destruction of two continents and the death of 70 million people barely worse for wear. This is a truly remarkable feat--perhaps one only possible in today's Exponential Age. The Tang never recovered from the An-Lushan rebellion; Central Asia never blossomed like it did before the Mongol conquests; no new Roman empire rose from the ashes of the old. But the Second World War was not a precursor to a new dark age. Under the old rules of static civilization--where wealth was not created, but taken--catastrophes of this scale required centuries of recovery before old heights could be reclaimed. The history of the post-war world dramatically illustrates that this is no longer the case.