On June 28, 1914, the assasination of Franz Ferdinand started a swirl of events that would lead to the outbreak of WWI.
Shortly after the assassination, Princip and his conspirators were all captured. The cyanide pills that they had been provided with their weapons for the job did not work and simply made them sick. Princip confessed everything and revealed their assassination details to the Austro-Hungarian investigators (so much for that oath...)
But the problem was that Princip claimed to have been provided with the weapons from members of the Black Hand that were Serbian intelligence officers. Most of the Black Hand leaders were just that, but they were not sanctioned any authority through the Black Hand to represent Serbia in any way. So was Serbia responsible? Many of Serbia's ambassadors to other countries started claiming that they did not order the assassination but were instructed to warn the Archduke of the impending threat that had been detected before his arrival in Bosnia on June 26th. Later it was determined that the Serbian ambassador in Vienna had failed to "correctly" relay the information of the threat (whatever that means...).
For Austria-Hungary, losing one's heir is about as big of a crisis as a monarch can have, and when an heir gets assassinated someone needs to pay for it. This was the perfect opportunity for Austria-Hungary to flex their muscles and assert their dominance in the Balkans and the Slavic population in their country (MacMillan, 554). Many of the advisers on Franz Joseph's staff both military and diplomatic urged immediate mobilization of troops, even if it was just for diplomatic purposes.
Adding to the weight of the decision of how Austria-Hungary would react, was the impact of how Germany insisted Austria-Hungary respond. Both of them were joined in alliance and Germany, in the years leading up to this, had established itself as a strong country and player in Europe. The German ambassador, Heinrich von Tschirschky, was telling the stressed officials that "If Austria-Hungary showed itself to be weak yet...Germany might have to look elsewhere for allies." By July 5th, Emperor Wilhelm II had promised Germany's full support. (MacMillan, 555 & 557)
But during the investigation of what happened in regards to the assassination, it became clear to Austro-Hungarian authorities that it would be impossible to determine if Serbia was truly behind it without going into Serbia. So Austria-Hungary put together an ultimatum with a list of demands that had to be accepted as a collective whole. On July 23rd, the ultimatum was delivered to the Serbian government with the understanding that they had until July 25th at 6pm to accept. In the next 48 hours, the Serbian government worked tirelessly to weed through and respond to each demand in the ultimatum. They came to the conclusion of accepting the entire demand except the two that gave Austria-Hungary the right to interfere in Serbia's internal affairs (i.e. the right to investigate the assassination within Serbia) (MacMillan, 571).
This is an understandable move on Serbia's part because what independent country would allow their sovereignty be violated in order to keep their neighbor happy? They did not want war, and frankly, Serbia was in no condition to wage war. It was an independent country and it wanted to stay that way. Serbia also had a slightly solid promise from Tsar Nicholas and Russia that they would come to their aid if war was inevitable. "Russia cannot allow Austria to crush Serbia." (MacMillan, 587)
There had been lots of trouble in the Balkans for years, so when news of trouble there reached the public no one was surprised or worried about a world war. But when Serbia refused to accept the ultimatum given by Austria-Hungary this started to key the public in that serious trouble was brewing in the East. The refusal to accept the ultimatum meant that many countries would be pulled into the depth of anticipation of who would attack who first.
If German/Austro-Hungarian forces invaded Serbia, Russia would step in to protect its Serbian allies. But because Russia was also allies to France and Great Britain, they too would be forced into the conflict to defend Serbian sovereignty.
Austria declared war on Serbia on July 28th, but no one moved.
Tensions were mounting as Russia started partial mobilization of its troops. Germany then warned Russia to stop mobilizing or they would be forced to mobilize their troops. A total of 10 telegrams would be exchanged between Wilhelm and Nicholas before August 1st; the two cousins were in competition with Germany being slightly more trigger happy than Russia. All of Germany was given the orders to mobilize for total war on July 31st (MacMillan, 611)
What had started as business between Austria-Hungary and Serbia was now the chatter in the background as the stronger members of the alliances took charge to ensure a short war ended with victory. This war would not be short and victory would come at a staggering price.