Following the assassination, Austria-Hungary approached Germany to see if they would support a war against Serbia. Germany agreed and on 28th July 1914, Austria-Hungary declared war on Serbia.

Austria’s ultimatum to Serbia was the decisive step ... we were well aware of the fact that eventual war operations on the part of Austria-Hungary might bring Russia into the field and might ...

Backed up by Germany, Austria-Hungary declares war on Serbia on 28 July. Russia quickly steps in to protect the small nation and mobilises its army. Germany responds by declaring war on Russia.

It was after the Kosovo war, which had begun between separatist ... annexation of Bosnia (home to many Serbs) by Austria-Hungary. Though outraged, Serbia was forced to accede.