Before the outbreak of world war 1 Serbia was fighting with the Ottoman Empire. They had to be a part of World War 1 because of an incident which occurred.
They have assassinated Ferdinand and this has to be the best excuse for having a war with Serbia!
When Franz Ferdinand, the heir to the throne of Austria-Hungary visited Bosnian capital, Sarajevo. The Black Hand which is a terrorist group who wanted to unite all the Serbs made two attempts to kill Ferdinand and in the second attempt, they were successful.
we don't want to have a war we are still recovering from the Balkan Wars we had.
we are not going to apologize for this assassination.
we received an ultimatum from Austria-Hungarian Empire. We accept all of it except, the 6th point in it.
The first attempt was throwing a bomb. But, it didn't work as they deflected. The second attempt was two gunshots by Gavrilo Princip. The first one hit the archduke, and the second hit his wife.
In Austria-Hungary, some of the politicians saw this assassination as an excuse to attack us and this was also a trigger to the World War 1.
Serbia couldn't agree to the six-point in the ultimatum as we couldn't allow Austro-Hungarian to be involved in the inquiry into the assassination. Serbian government promised to hold an inquiry but couldn't accept Austro-Hungarian involvement in this.
On July 28, 1914, one month after Archduke Franz Ferdinand was assassinated, Austria-Hungary declared a war against us, this was the beginning of the First World War.