What you have to understand that the major powers within the Balkan Territories were beginning to mobilize but took some time to do. With both the huge Axis and the Triple Entente they needed a reason to fabricate a declaration of war on someone. The invasion of Serbia right after the assassination caused the Serbs to call upon their huge allies the Russians.This was in accordance with a defensive pact they had with each other. The Austrians now fearing things could easily get out of control called on their allies (Germany) which then unfortunately caused a domino effect. Soon after each major power was pressured to join in on either side. http://www.firstworldwar.com/features/balkan_causes.htm

Diplomatic tensions had been building in the region for quite some time, as well as across Europe. Ever since the Franco-Prussian War Germany and France had been quite wary of one another. France created a line of forts along their border with Germany, while the Germans perfected the Prussian system of Mass Conscription. Germany also began regular drilling of conscripts to make them into an effective fighting force. Meanwhile this causes tensions with Russia, which entered into an alliance with France in 1892. In order to protect themselves from Russia, Germany entered into the Dual Alliance with Austria Hungary to provide a more secure Eastern Front.

Because of this, German Military Heads knew that when tensions boiled over with France again (they all believed it inevitable) that Russia would get involved, thus making a two front war. Serbia was also a bitter enemy of Austria and a great ally of Russia, which made a chain of alliances that would be disastrous.

Early in the 20th Century, there were many crises in diplomacy. However, when each happened none of the actors in the war was willing to trigger the conflict without the support of their allies, so basically the larger crises occurred before the time was come for the war to break out. But with each tensions mounted even higher, building to a breaking point that burst into flames with the assassination. Austria Hungary declared war on Serbia because they felt they had the justification for it, and thought Russia would not come to Serbia's aid (which is why they worked so hard to connect the government of Serbia to The Black Hand, which carried out the assassination). But Russia came anyway, and after that what happens is pretty obvious.

Think of the term MAIN or MAINA M-Militarism being the first of this. It has to do with the imperial powers and how a life in the military often brought great fame and wealth to those in need. It provided cheap labor while keeping the poor both fed and clothed for the most part. A-Alliances as I already stated above had were always constantly changing as the the nations always looked for and chance to bully and devastate their neighbors. I- Imperialism is another one that had to do more with the allies and Germany rather then Russia and Austria. As you know both France, Germany, Britain, and Spain were all trying to carve up new territories and colonies. This specifically was more significant after the war as the allies dismantled and demanded repayments upon the Axis. The Ottomans joining towards the middle of e war was completely destroyed as Syria and Turkey were carved up. Germany after the way was forced to all of their holdings in Africa and parts of what is now New Zealand. N- Nationalism. This was a major factor because till up to this point ever nation fought wars with primitive technology. Wars were often fought in smaller nations as this often lead to annexation or vassalage as economically speaking they were destroyed. A-Assassination was just the fabricated casus beli or cause for war that lead to the escalations of the great war.

All in all there was not one single factor but many that come with long term alliances and defensive pacts