Austria - Hungary sends a threatening ultimatum to Serbia...
Austria-Hungary sent Serbia an ultimatum - a list of things they had to do. It included letting Austria- Hungary carry out an investigation into the murder of the Archduke.
Doing what Austria told them to do would make Serbia look weak and they were offended by the tone of the ultimatum. But Serbia's friends Russia, France and Britain warned Serbia about the consequences if they didn't do what Austria wanted.
...and declares war.
Serbia agreed to do almost everything Austria had asked for but on the 28th July Austria - Hungary declared war on Serbia, supported by Germany.
Franz Conrad von Hötzendorf was in charge of the Austro - Hungarian military. He supported the idea of attacking Serbia because he thought it was important for the Austro - Hungarian Empire to prove it was still a force to be reckoned with.