The rise of Nationalism (the Nation-State) that resulted in a race for resources via Colonialism during the Nineteenth Century led to global scale conflict in the Twentieth.  The reaction from those under the yoke of colonial governments also contributed to the blood shed.  Also, the rise of structured ideological movements paired with Nationalism resulted in social change on a grand scale.  The resultant power vacuums created from the crumbling colonial systems and the introduction of Marxist ideologies that vied to fill that vacuum, are the primary reasons for the upheavals that plagued that century.   

When you add in the drastic improvements in technology (which you forbade in this discussion), it made killing so much more efficient, the statistics had to become staggering.

@donaldbaker Pretty good answer.  Would you say that "globalism" also contributed?  I say this in a broad sense, including:

• international alliances: a relatively local assassination spiraled into global war due to national agreements - Germany-Austro-Hungary, Serbia-Russia, etc.; and
• international trade: Germany was utterly crushed under the ridiculous terms of Versailles because of the arrangements which killed its ability to make war payments to France/Britain, which in turn hurt their ability to repay their loans from the U.S.  The economic ruin in Germany allowed Nazism to rise and a new Germanic "savior" to emerge.

But if globalism contributed to the rise in death during the century, it must be true that less globalism would lead to fewer deaths.  But is this the case?  I think the answer is yes.  While globalism has clear economic benefits, the downside is that one nation's problems become other nations' problems.  This leads to much more expansive wars when they do emerge.

You did have the most evil tyrants in that century: Lenin, Stalin, Mao Zedong, Hitler and some others.  Each of them were responsible for millions.

Posted by: @skiguy

You did have the most evil tyrants in that century: Lenin, Stalin, Mao Zedong, Hitler and some others.  Each of them were responsible for millions.

Yeah but don't you think there were people equally as evil - if not moreso - who lived in prior centuries, who simply did not kill as many? 

I'm sure there were, but they didn't kill 20 million people like Stalin did.

This is kind of a chicken-and-egg scenario; does the most evil person kill the most people, or is the person who killed the most people the most evil?

Anyway, my thinking was that people in prior centuries didn't kill as many people because they couldn't, and not just because of lack of killing technology. My view is that without formulating an underlying, all-encompassing ideology, evil leaders would not be able to persuade followers to go through with mass executions.  Of course, one way to spread ideology is through mass media, which grew by leaps and bounds in the twentieth century through radio and film.

How many died from religious persecutions though?  Religion is a more powerful driver than even ideology in my opinion.

That's a good point, although at the same time, I think it would be hard to come up with a war that was motivated primarily by religion, rather than politics.  In other words, "religious wars" typically have some political basis, so religion becomes a kind of pretext.  We discussed this over at WCF some years ago, and at least that is the line of thinking we discussed.

However, I think the increase in deaths was also because of the idea of "total war", in which general populations were targeted, which did not come about until World War I.  With World War I, and the increased use of civilian populations to help the war effort, these civilian populations needed to be stopped (e.g. British blockade of Germany, helping lead to the Turnip Winters of 1916-17).  In the twentieth century, war had become a nation-wide affair which required even women and children to participate in one form or another.

When total war enters the mindset of warring countries, this opens up nations to engage in genocide.  Thus, the Armenian genocide of 1915 by the Turks, and then the later systematized genocide under the Nazis.