On June 28, 1914, a member of the revolutionary group Young Bosnia assassinated Archduke Franz Ferdinand. The archduke was the presumptive heir to the throne of Austria-Hungary, which had annexed Bosnia-Herzegovina several years before. Young Bosnia was trying to overthrow Austria-Hungary’s rule of the region and had acquired weapons for a revolution with the help of Serbian nationalists. Austria-Hungary discovered this connection and declared war on Serbia a month after the assassination.

The reason this regional conflict escalated into World War I—which killed some 20 million soldiers and civilians—is something people have been debating since it ended. Though there’s no one answer, part of the reason the war grew so large has to do with a complicated network of alliances that European nations made with and against each other in the decades leading up to the war.

These alliances created a balance of power in Europe that some hoped would actually prevent war. Yet for many countries, the alliances made them feel like they had no choice but to join a growing international conflict.

World War I had two main factions: the Central Powers, which included Germany, Austria-Hungary, Bulgaria and the Ottoman Empire; and the Allies, which included France, Great Britain, Russia, Italy, Japan and later, the United States. With the exception of Italy—which switched sides—most of these alliances had roots in prewar agreements.

A lot of the animosity behind the two World War I camps dates back to the Franco-Prussian War of 1870, in which German states (led by Prussia) thwarted France’s attempt to reassert dominance on the continent. After the war, Prussia and other German states united to form the German Empire, and allied itself with neighboring Austria-Hungary. In 1882, the newly-unified state of Italy joined Germany and Austria-Hungary, forming the Triple Alliance.

Over the next few decades, European empires continued to create alliances and informal agreements as they competed for power and colonial territories around the globe.

“France was looking for an ally against Germany,” says Jonathan Casey, director of archives and the Edward Jones Research Center at the National WWI Museum and Memorial. In the 1890s, France formed an economic and military alliance with Russia, whose empire posed “a threat to Germany’s eastern border.”

Both France and Russia gradually established good relations with France’s historic rival, Great Britain, resulting in the Triple Entente between the three nations. This “entente,” or understanding, was not a military alliance, but it did help establish competing camps in Europe.

“The alliance system was basically an attempt at trying to form collective security,” says Richard Fogarty, a history professor at the University at Albany. “But what it meant was it replaced a system in which a whole bunch of states competed with each other individually with a system in which states competed with each other in, ultimately, two major alliances. And that set the stage for the war.”

After the assassination of Archduke Ferdinand came the July crisis. During this month, Austria-Hungary issued a list of demands to Serbia. Serbia, knowing that it could expect support from Russia as a fellow-Slavic state, refused to give in to one of the key demands. Austria-Hungary confirmed with Germany that it would receive its support if it went to war, and Russia did the same with France.

“I think most of the people involved either thought or hoped that by saying and knowing that they had the support of their allies they could actually avoid war,” Fogarty says. “In other words, ‘Austria won’t dare to do anything to Serbia if it knows Russia’s behind it.’ And then the Austrians think, ‘Russia won’t dare to do anything if they know the Germans will get involved’”—and so on.

But on July 28, Austria-Hungary declared war on Serbia. Over the next week and a half, Germany declared war on Russia and France, Britain declared war on Germany, and Austria-Hungary declared war on Russia.

The justification that Britain gave for entering the war didn’t even have to do with its Triple Entente with France and Russia—it had to do with the much older Treaty of London (also called the First Treaty of London) from 1839 that mandated Belgium should always be a neutral state. When Britain entered the war, it argued that Germany had violated this treaty by invading Belgium.

Over the next four years, many other nations entered the war. Japan joined the Allies at the end of August 1914. In November, the Ottoman Empire joined the Central Powers. The next year, Italy joined the war to fight against Germany and Austria-Hungary, with whom it had previously formed an alliance. In 1917, the United States joined the Allies.

The 1919 Treaty of Versailles that dictated the terms of the war’s end placed most of the conflict’s blame on Germany. But the reality of why so many nations became involved in the war is much more complicated—and something that people have been debating ever since.

As Fogarty says, “What happens at the end of the war is everybody says, how the heck did that happen?"