The Great War or/aka World War One was directly triggered by the assassination of Franz Ferdinand (the Austrian archduke) and his wife (Sophie, Duchess of Hohenburg) in 1914 by Gavrilo Princep, who was a Bosnian revolutionary.

However many historians agree that even if Franz Ferdinand was not assassinated, World War One was still very likely to happen because of several reasons and conflicts. Let's ignore that, and focus on the events after the assassination that led to the initial start of the conflict:

Ferdinand's assassination by a Serb nationalist (Princep) gave the Austria-Hungarian Empire an acceptable excuse to attack its neighbour and enemy, Serbia.

Before attacking Serbia blindly however, the Austria-Hungarian Empire made sure it had the backing of Germany, who they had a pact/treaty/alliance with. On the other hand, this allowed Serbia to get the backing of their ally, Russia.

Russia had an treaty with both France and Great Britain.

This meant that when Austria-Hungary officially declared war on Serbia in July 28, 1914, much of Europe had already become a part of the dispute.

And so forth, World War 1 began between two imposing sides: The Allied Forces (Russia, GB and France) against the Central Powers (Austria-Hungarian Empire and Germany).

I hope I gave a detailed explanation.