As the summer of 1914 approached, the balance of power in Europe looked shaky at best. It would take only a single crisis—the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, and his wife Sophie Chotek by a young Bosnian Serb nationalist in Sarajevo—to push the continent’s six major powers into World War I, which devastated the continent and killed some 17 million soldiers and civilians.

But for all its historic importance, Franz Ferdinand and Sophie’s deaths might not have happened at all, if it weren’t for an odd series of events and decisions—and a wrong turn—that placed the royal couple squarely in the path of their assassin’s gun.

In addition to being the heir to his uncle’s throne, Archduke Franz Ferdinand was also inspector general of the Austro-Hungarian Army, which had decided to hold its summer military exercises in Sarajevo, the Bosnian capital.

Back in 1908, the dual monarchy of Austria-Hungary had annexed Bosnia and Herzegovina, a region that had previously been under the control of the Ottoman Empire. Home to a large Slavic population, Bosnia and Herzegovina had nationalist ambitions of their own, but nearby Serbia wanted to incorporate them into a pan-Slavic empire.

Wary of Serbia’s ambitions for territorial expansion, Austria-Hungary had sought and received assurances from Germany that it would stand behind the dual monarchy in case of war with Serbia (and Serbia’s powerful ally, Russia). By choosing to hold its military exercises in Sarajevo in June 1914, and to send the heir to the throne to oversee them, Austria-Hungary intended to make a show of force to warn Serbia against any further expansion and aggression.

In the years leading up to WWI, a series of agreements between the powers of Europe helped determine where and when battlelines were drawn.

June 28 was a particularly significant date for Serbia: It was St. Vitus’ Day, the anniversary of the Serbian defeat in Kosovo by Ottoman forces in 1389, and this would be the first celebration of the occasion since Serbia had won back Kosovo in the Second Balkan War.

For their part, Serbian nationalists saw the archduke’s visit to Sarajevo on this of all days as an unforgivable insult—and they sought to strike back.

Despite warnings of possible terrorist attacks during the visit to Bosnia, few official security precautions were taken. Franz Ferdinand and Sophie traveled in an open car, and the route their motorcade would take through Sarajevo had been made public well beforehand.

On the morning of June 28, seven Bosnian Serb members of the separatist group Young Bosnia, acting with assistance from a Serbian nationalist group called the Black Hand placed themselves along that route. They had strapped explosives to their bodies, carried loaded revolvers and were all equipped with cyanide so they could commit suicide rather than be caught.

As the motorcade rolled along the Appel Quay, a major street running through the center of Sarajevo, a Bosnian Serb named Nedeljko Čabrinović threw a bomb toward the archduke’s car. The driver managed to accelerate out of the way, but the bomb hit the vehicle behind, injuring several people, including the adjutant to General Oskar Potiorek, governor of Bosnia.

Though Čabrinović took his cyanide and threw himself into the nearby river, the poison didn’t work, and the river was too low for him to drown, so he was quickly arrested.

“We’re entitled to ask ourselves why, at this point, the archduke didn’t simply call the visit off,” Christopher Clark, a professor of modern European history at the University of Cambridge and author of The Sleepwalkers: How Europe Went to War, told NPR’s All Things Considered in 2014.

“That was proposed by some members of his entourage,” said Clark, “but he hated being told what to do. He was a very irritable man, and he said ‘don’t be ridiculous.’”

Instead, the group continued on to Sarajevo’s city hall, where they met with dignitaries including the mayor, who failed to alter his prepared speech about the happy and “enthusiastic” greeting Sarajevo’s citizens were offering to the archduke.

As Clark recounted in his book, Franz Ferdinand furiously interrupted the mayor’s speech, exclaiming, “I come here as your guest and your people greet me with bombs!” before his wife Sophie was able to calm him down.

After Franz Ferdinand made his own speech and tended to some official business, he wanted to visit the injured adjutant in the hospital before leaving town.

For security reasons, it was decided that the motorcade should proceed out of the city via the Appel Quay, rather than take its planned route along Franz Joseph Street and into the narrow streets of Sarajevo’s bazaar district.

Unfortunately, the drivers didn’t pick up on this changed itinerary. “They’re talking about this in German, and the driver of the first car is Czech, and so is the driver of the second car,” Clark told NPR. “They don’t understand what this conversation’s about, and nobody bothers to translate for them.”

As a result, the first car turned onto Franz Joseph Street, followed by the second car, carrying Franz Ferdinand, Sophie and Potiorek. Amazingly, this wrong turn took them right to where 19-year-old Gavrilo Princip had stationed himself along the originally published route for the motorcade, under the awning of a general store.

(It’s probably not true that Princip had stopped to get a sandwich, as one popular myth about the assassination goes.)

As Potiorek yelled at the driver that he had taken a wrong turn, the car slowed to a stop right in front of Princip, who fired two shots into the car, hitting Franz Ferdinand and his wife at point-blank range.

“If Princip had spent his entire life learning about human anatomy, he couldn’t have placed his shots better than he did,” Clark said. “They were both lethal.”

The son of a Bosnian farmer, Princip had tried to enlist as a Serb guerrilla in 1912 when the Serbs were fighting the Ottoman Empire, but he was rejected as too small and weak.

As a student in Belgrade in 1914, he and several other earnest young ultra-nationalists (including Čabrinović) decided to try and win a victory for their cause by assassinating the archduke during the planned visit to Sarajevo. Armed by connections in the Serbian military and the shadowy ultra-nationalist organization the Black Hand, Princip and his fellow assassins headed to the Bosnian capital.

In addition to Čabrinović and Princip, several of the other young terrorists had opportunities to act against the royal motorcade but backed off.

“They were scarcely more than boys, really, very inexperienced,” Clarke said. “They simply froze with terror as the car approached. One of them ran away, another one just remained stock-still, unable to move.”

In the aftermath of Franz Ferdinand’s assassination, Princip, Čabrinović and most of the other conspirators were arrested and tried in Sarajevo. Because he was under 20 years old, too young to be executed under Austro-Hungarian law, Princip received a sentence of 20 years imprisonment.

In 1918, Princip would die of tuberculosis in Theresienstadt, a prison in northern Bohemia which, years later, would be used by the Nazis as a concentration camp in World War II.

After that fateful wrong turn, a young student’s two gunshots in Sarajevo provided the necessary spark that would upset the fragile balance of power in Europe and send the world to war. On July 28, 1914, one month after Franz Ferdinand’s death, Austria-Hungary declared war on Serbia, beginning a chain reaction that would lead to four years of horrific conflict with millions of people dead.