Why did austria-hungary declare war on serbia during WWI?

For this answer, you need to not look nearly as much at politics as you do religious discord. Read chapter five of the Qu'ran and understand that it teaches that Christians and Jews are corrupt in their faith and that a Muslim should never have dealings with a Christian or a Jew. The Arch Duke was Catholic. The "Black Hand" was Islamic.

RE:

Why did austria-hungary declare war on serbia during WWI?

Okay, so i have to write a paper on Austria-Hugary in WWI, and I get the whole "Black Hand" stuff, and how the assassination of Franz Ferdinand basically started the war, but what happend before all of that with austria-hungary? Like why did Austria-Hungary feel the need to declare war/try to take...

It explains it here, but Serbian military officers stood behind the assasination (which was an attempt to get parts of Austria-Hungary to break off and join Serbia), therefore Austria blamed the state of Serbia.

For this reason, AUstria felt obliged to at least send out a punitive force - in other words, an army to punish Serbia - but since 1870, her main targets of territorial expansion had been in the Balkans. In keeping with these targets, the 'punitive force' plan turned into a 'conquest' plan.

The war between Austria-Hungary (AU) and Serbia was inevitable since the politics of Central Europe was heading into this conflict from the beginning of the 20th century. Serbia gained independence from Ottoman Empire and it was growing in strength that would eventually undermine multinational system of AU monarchy. Dominant Germans were indeed minority – 25% or so, and the rest were subjected nations with various degrees of love and loyalty toward Vienna. AU was aware of explosive nationalistic feeling within its domain, and often pitted nationalities against each other like Poles against Czechs or Poles against Ukrainians. The existence of small, but vital state in Balkans was a threat to AU Empire and it could provide a model for small nations how to gain an independence from AU. Between 1900-1910 many Slavic people were migrating for jobs into Balkans to build railways, industry, or banking within AU, and there was significant revival in interest of south Slavic history, language, and statehood within monarchy. The German dominated AU strongly disliked Serbia, and it was perceived as a threat to the monarchy. AU did not have colonies, and it believed that the Balkans was its backyard. It did invest heavily into defeat of the Ottoman Empire and all what it got was poor Bosnia, while Serbia became boulder that blocked Austrian expansion into Ottoman territories.

Right prior outbreak of WWI, Balkans was involved in series of mutual and destructive war, and the nationalistic feeling was high in Bosnia that possesses large Serbian population. The AU and Germany tried to secure Balkans for past 40 years with series of diplomatic manipulation and push Russia out of the region. Bulgaria was pro German friendly state, so was Romania that initially had treaties with Germany. Only Serbia was unbowed and was following its own diplomatic course that was against politics and wishes of AU and Germany. AU was looking for a while how to eliminate Serbia, but it needed reason. Assassination of Ferdinand was a prelude to war, but without it, AU would find another reason. AU believed that Serbia was weakened by series of Balkan wars and it would be easy pick to conquer and destroy. The mood was in Vienna that the war would be over before plums will be ripe (which in Central Europe is the end of the summer-September).

With the war in mind in summer 1914, Serbia was hoping to get Bosnia and its Serbian population under its control. AU was looking to eliminate Serbia and extent its boundary toward Bulgaria and integrate western Balkans with its domain. It also wanted to prove to unhappy Slavs (Czech, Poles, Slovaks, Slovenians, Croatians, and Ukrainians) within its domain that the Germany and German dominated AU will not tolerate any independent Slavic state anywhere in Europe. When the war was declared, Germans across the monarchy was rejoined, but the rest of the nationalities were not eager to fight Serbia.