The murder of the heir to the Austro-Hungarian Empire and his pregnant wife produced widespread shock across Europe, and there was initially much sympathy for the Austrian position. The Austrian Government in Vienna opportunistically saw this as an opportunity to settle the perceived threat from Serbia once and for all.

After conducting a criminal investigation, as well as verifying that Germany would honor its military alliance, Austria-Hungary issued a formal letter to the government of Serbia. The letter reminded Serbia of its commitment to respect the Great Powers' decision regarding Bosnia-Herzegovina, and to maintain good neighborly relations with Austria Hungary. The letter also contained specific demands aimed at destroying the funding and operation of terrorist organizations which arguably had perpetrated the Sarajevo outrage.

Serbian reservists being transported on tramp steamers on the Danube, apparently accidentally, crossed on to the Austro-Hungarian side of the river at Temes-Kubin and Austro-Hungarian soldiers fired into the air to warn them off. This incident was blown out of proportion and Austria-Hungary then declared war and mobilized its army on July 28, 1914. Under the Secret Treaty of 1892 Russia and France were obligated to mobilize their armies if any of the triplice mobilized and soon all the Great Powers except Italy had chosen sides and gone to war.

It could be argued that this assassination set in train most of the major events of the 20th century, with its reverberations lingering into the 21st. The Treaty of Versailles at the end of the First World War is generally linked to the rise of Adolf Hitler and World War II. It also led to the success of the Russian Revolution, which helped lead to the Cold War. This, in turn, led to many of the major political developments of the twentieth century, such as the fall of the colonial empires and the rise of the United States and the USSR to super-power status.

However, if the assassination had not occurred, it is very possible that European war would have still have erupted, triggered by another event at another time. The alliances noted above and the existence of vast and complex mobilization plans that were almost impossible to reverse once put in motion made war on a huge scale increasingly likely from the beginning of the twentieth century.