Prof. Michael Neiberg of the U.S. Army War College in Carlisle, Pa., said some blame those who held political power at the time, and their divergent systems of government, while others insist it's difficult to assign blame at the feet of any one culprit.
"If anybody goes looking for simple causes, they're going to either be disappointed or they're going to reduce the history so much that it won't make sense anymore — 1914 was an unbelievably complicated world," said Neiberg.
Europe was a divided region in August 1914. Britain, France, and Russia formed the Triple Entente, while Germany, Austria-Hungary and Italy made up the Triple Alliance.
Many view the assassination on June 28, 1914, of Archduke Franz Ferdinand at the hands of a Serbian nationalist as the spark that ignited the conflict.
The heir to the Austrian throne and his wife were gunned down in Sarajevo.
About a week later, Austria-Hungary received a pledge from Germany, known as the "blank cheque," for any support necessary to deal with Serbia.
But on July 21, Russia, an ally of Serbia, warned Austria-Hungary against taking action against the Balkan state. Nonetheless, just two days later, Austria-Hungary presented Serbia with an ultimatum and set a 48-hour deadline.
Serbia's response was rejected, and Austria-Hungary declared war July 28.
On July 31, Russia mobilized its troops, and the next day Germany declared war on Russia. Within days France, Belgium and Britain were bought into the war.
Historian Margaret MacMillan, a professor at the University of Oxford and author of The War That Ended Peace: The Road to 1914, said the decisions — made or not made — in those final days are significant in order to understand the conflict's origins.
"When you get right down to the summer of 1914, you have to look at the human decisions and the human errors that were made, and I think, until very, very late on, they could have avoided a war, and they could well have passed through 1914, another moment of crisis, and not had a war,” she said in a recent interview.
In her book, MacMillan explored why a century of relative peace didn't endure in Europe considering there had been no major continental conflicts since 1815.
MacMillan said tensions were escalating between the two power blocs, and some nations wanted to expand their territory at the expense of others, especially in the Balkans. Russia was growing and developing with speed, which worried Germany.
Heightened nationalism existed in Europe, as well as a "psychological drift" in which decision-makers thought war could be useful, she added.
"Whenever there was a crisis, there'd be talk of a general war in a rather sort of casual way," she said. "You know, people would say, 'when the war comes,' rather than 'if a war comes,' and so I think you get a sort of psychological acceptance, in some circles, that war might be something good."
Systems of government also complicated matters. In Britain, cabinet had to approve a declaration of war, while in Germany and Russia, the kaiser and the czar, respectively, had sole authority over foreign policy and the military. As a result, power was concentrated in the hands of people who weren't necessarily fit to wield it, MacMillian said.
Prof. William Philpott of King's College London says the lack of an international body that was monitoring global conflicts also contributed to the chain of events.
"There was no collective security model that we had firstly in the League of Nations after the war, and later in the United Nations, restraining or at least tempering that sort of international crisis," he said.
What should have been a regional war in the Balkans, therefore, became a widespread European conflict, Philpott said.
Russia was humiliated during a crisis in Bosnia in 1908, and a dispute over Morocco in 1911 exposed a growing rivalry between France and Germany, he said.
But war could still have been averted had it not been for Austria-Hungary's stance against Serbia, he said.
"So in some ways it's the nexus of policy between Austria and Russia with Serbia caught in the middle that is the significant factor, I think," he said.
Philpott said Russia could have accepted that Austria was right, and Serbia was wrong, over the assassination of the archduke, and opted against backing Serbia so strongly. But there were forces in Russia who felt Serbia needed support.
Neiberg, of the U.S. Army War College, said the archduke's assassination didn't worry many major powers but the reaction of Germany, Austria-Hungary and Russia created problems.
The assassination made Austria-Hungary the victim, not the aggressor, he said.
"They make up their minds that the circumstances the assassination created gives them the perfect cover to launch a pre-emptive war," said Neiberg. "The second problem is that instead of telling them not to do it, the Germans encouraged them to do it."
The true crisis was sparked when Austria-Hungary presented Serbia with the ultimatum, said Neiberg.
"July 23 is the real bombshell," he said. "It's why we call this period the July Crisis, and not the June Crisis. So when everybody sees that ultimatum, then they know how serious the situation in Europe is."
Then Russia mobilized, Neiberg said, leaving Germany believing it could do what it needed to do to defend itself. The average German citizen honestly believed Russia was threatening invasion.
And so Germany mobilized its armies to defend itself against Russia, but seven of eight German field armies invaded Belgium, France and Luxembourg under war plans developed years earlier.
"Once you get Britain and France in it, you get the full force of the British and French empires. You get everything from Vancouver to Cape Town to Sydney to Auckland to India. You get all of that in," he said.
The crisis unfolded "unbelievably quickly" over the span of a week, Neiberg added. That was by design as Austria-Hungary forced the timetable forward.
MacMillan doubts historians will ever settle the debate of what or who is to blame for the First World War, and that's probably for the best.
"We shouldn't be trying to tell even schoolchildren that there is just a simple explanation of the war," she said.
"I think we should tell them that ... there's a discussion, and that it's all right to have different views on what happened in history. Some things you just won't get a consensus."