This weekend marks the 100th anniversary of an obscure event that changed the world -- the assassination of Archduke Franz Ferdinand in Sarajevo. A month later, five of the six great powers of Europe had declared war on each other.
By the end of the conflict four years later, 37 million soldiers and civilians were killed or wounded, empires were reshaped, and society had changed forever. Victorian principles of humankind's orderly progress and the gallantry of war were upended. In its place came the barbarity of trench warfare, gas attacks, starvation and the great "No Man's Land" between armies.
"The First World War revealed that barbarism lay just beneath the thin veneer of civilization," said Gordon Martel, author of the new book "The Month That Changed The World: July 1914." "It revealed that if you scratch the surface that we are all killers under the skin. And we all fear death."
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But what if the Archduke's assassin -- a Serbian nationalist -- missed? What if the "Great War" hadn't occurred? How would the 20th century have evolved differently? Without World War I, there probably wouldn't have been World War II. No Hitler. No Holocaust. No Cold War.
Without tens of millions of deaths, European nations would have likely put more resources into building their economies. Germany would have become an economic, scientific and cultural powerhouse.
The United States would have remained more isolated, less intertwined with the rest of the world, and also less tolerant of the rights of women, blacks, Jews and other minority groups. There wouldn't have been a President Barack Obama, nor a President John F. Kennedy.
At the same time, there wouldn't have been nuclear weapons, computers and possibly even the Internet. Why? Military spending drove all of these technological advancements.
These are some of the scenarios sketched out by historian Richard Ned Lebow, professor of international political theory at King's College London and author of the 2014 book "Archduke Franz Ferdinand Lives!: A World Without World War I."
Lebow says that one important consequence of both world wars was greatly accelerated science and technology through government support of research and development of weapons. Without the military's need for airplanes, safe airline travel would have been delayed several decades as well, according to Lebow.
"Antibiotics would have been delayed, and we wouldn't have had an information revolution," Lebow said in an interview. "In the United States, maybe the best thing about the second half of the 20th century was development of tolerance. The U.S. before 1945 was a pretty backward and racist place."
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Blood transfusions, sanitary napkins and antibiotics all came out of the medical needs of World War I, Lebow said.
World War I also made room for women in the workplace to fill in for soldiers at war. When the fighting stopped, it led to greater political activism, including the right of women to vote. That would have happened much later, Lebow said.
Lebow's book of counterfactual history postulates a world of longer-lasting European empires that are less tolerant of democracy, as the Ottoman, Austro-Hungarian and Russian empires remained in power.
Germany would not have been punished at Versailles, and Hitler would not have had grievances that led to his later rise to power. Jews in Europe would have prospered and increased in population; there might not have been a state of Israel, Lebow writes.
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The guessing game of "what-if?" has other outcomes as well. Jack S. Levy, professor of political science at Rutgers University, believes that even if the Ferdinand escaped his death in Sarajevo on the morning of June 28, 1914, something else would have started World War I. For one thing, he says that Germany was fearful of Russia's increasing power, which it believed threatened its own security.
"We cannot know for certain what would have happened," Levy said. "But my best guess is there probably would have been a war anyway."