Effect : Austria-Hungary mobilized for war
When Serbia responded to Austria-Hungary's ultimatum, they accepted all the conditions save one. Austria-Hungary immediately declared war on Serbia, which had always been their intention given the demeaning demands they had made.
When Serbia rejected the Hapsburgh ultimatum, Russia supported Serbia, Germany backed the Austro-Hungarians, who invaded Serbia bringing the triple alliance, (Russia England France) into the First World War. The Ottoman Empire backed Germany and Austro-Hungarians to create the Axis. Italy backed the triple alliance to create the Allies, which defeated the Axis, in 1918. Canada, Australia, New England and other Commonwealth Nations supported England, as they did in WW2.
to fight for them
Serbia agreed to all but one of the ten demands. The one that they could not accept (#6) demanded that Austrian police be allowed to operate in Serbia as they conducted arrests of suspects in the assassation of the Archduke.
The Hasburg Ultimatum was a document sent to Serbia from Austro-Hungary giving them terms to accept to prevent war, after a Serbian Terrorist Organization shot and killed the air to the throne to Austro-Hungary. Upon Serbia's refusal to sign the Hasburg Ultimatum, Austro-Hungary declaired war on Serbia, and invaded it. This was the spark to World War I, as immediatly after, Austro Hungary's ally Germany invaded Russia, Serbias ally.
It was Germany saying that no matter what the Austro-Hungarian Empire decided to do with Serbia with they will support them.
It's clause number 6: "To take judicial proceedings against accessories to the plot of the 28th of June who are on Serbian territory; delegates of the Austro-Hungarian Government will take part in the investigation relating thereto;"Serbian answer to that clause was:"..As regards the participation in this inquiry [which Serbia intends to hold] of Austro-Hungarian agents... [Serbia] cannot accept such an arrangement, as it would be a violation of the Constitution..."
Serbia
The Austro-Hungarian Empire declared war on Serbia.
(Part of) The Austro-Hungarian Empire, Serbia, and Monte-Negro
They declared war on Serbia.
Enemies in the "Great War " were : - Austro Hungarian empire - Germany - Ottoman empire - Bulgaria