It was from Austria.
austria - hungary
He was an Archduke of Austria. and was assassinated in Serbia.
Austria-Hungary feared Serbia because of the growing size of Serbia and because of Serbia existing as a free national state Hope this helps!
Austria-Hungary believed that Russia would not come to the aid of their ally Serbia because they thought Russia was preoccupied with internal issues and would not risk a war with Austria-Hungary.
Austria sent the ultimatum to Serbia on July 23rd 1914
Austria-Hungary, Mentenegro, and Serbia
Because serbia didn't want to agree to everything Austria Hungary wanted they invaded serbia which would force Serbia to fight back.
The Archduke of Austria Franz Ferdinand was assassinated by a Bosnian-Serb. In retaliation Austria-Hungary gave Serbia an Ultimatum of demands designed to destroy Serbia's influence in Bosnia. Serbia rejected the Ultimatum and Austria-Hungary declared war on Serbia. Germany was Austria-Hungary's ally and agreed to assist Austria-Hungary in the invasion of Serbia. Russia, fearing an invasion of Serbia would end it's own influence in the Balkans began to mobilize for war. As a result, Germany declared war on Russia.
Serbia partially accepted the ultimatum from Austria-Hungary but rejected some of the demands. This led to Austria-Hungary declaring war on Serbia, which ultimately sparked the beginning of World War I.
Austria-Hungary's future king was assassinated.
The 1st country to declare war on Serbia was Austria-Hungary.