23rd July 1914
Germany could have refused to back Austria on the grounds that the ultimatum to Serbia was too harsh. ;)
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.
It wasn't directly a country that started WW1 but there wasn't a specific event that caused it either, it was just a build up of events. One of the most common and one of the biggest events that happened before WW1 was the assassination of Arch Duke Franz Ferdinand.
Nicholas II did NOT welcome World War I. It was only after Austria tried to overrun Serbia that Nicholas went to Serbia's aid and was given an ultimatum by Germany, that Russia entered the war.
Austria issued Serbia a 10-part ultimatum in response to the role of Serbian and Slavic nationalists in the assassination of the Austrian archduke.
Austria sent the ultimatum to Serbia on July 23rd 1914
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.
Serbia
After Serbia's reject of Austrian Ultimatum, on July 25, 1914, the Russian government ordered to start the partial mobilization of its armed forces.
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.
After Serbia's reject of Austrian Ultimatum, on July 25, 1914, the Russian government ordered to start the partial mobilization of its armed forces.
23rd July 1914
the germans
Because the the Archduke Franze Ferdinand was the heir to the Austria-Hungary throne when him and his wife when assassinated by a Serbian Nationalist in Sarejevo. Austria Hungry Gave Serbia a ultimatum and when Serbia did not follow the ultimatum Austria-Hungry declared war with Serbia
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.
Germany could have refused to back Austria on the grounds that the ultimatum to Serbia was too harsh. ;)