Explain the role of nationalism, imperialism and social Darwinism as causes of World War One.
Serbian nationalism and the desire to throw off the influence of the Austro-Hungarian Empire directly influenced Gavrilo Princip, the assassin who killed Austrian Arch-Duke Franz Ferdinand and set off the chain of the events that led to World War One.
The ultimatum that Austria made against Serbia in response to the assassination was driven by the Austrian's desire to show strength and to intimidate little Serbia, but most of all, the demands the Austrians made, which they knew Serbia could never accept (including the loss of sovereignty) were directed at the rest of the Europe, to show everyone that Austria was still strong and proud, and could not intimidated. Meanwhile, German nationalism and militarism fueled that country's desire to take the spot of "top dog on the continent" away from Great Britain. It was that desire that led Germany to (mistakenly) calculate that a quick war could achieve such a shift in the so-called balance of power.
As a result of this thinking, German's military leaders made the grave mistake of issuing Austria the now famous "Blank Check" of support. Had Germany, Austria or Serbian nationalists been slightly less proud and less adamant about preserving their national dignity above all else, the war would have been averted. Yet Germany, England and France were driven not only by nationalism, but also by the desire to expand their colonies (in Africa and elsewhere) at the expense of each other.
Germany hoped to seize certain French colonies in the north Africa, as well as British colonies in the Middle East. Conversely, France wanted and eventually got some German colonies in Africa, as well as territories in Europe, such as Alsace-Lorraine and for a time, the Sudetenland. Moreover, Russia, Germany and Austria all hoped to gain territory in Eastern and Central Europe. The big prizes there were Poland and Czechoslovakia. This 19th Century notion of wars of aggression, from which the victors could gain land and therefore wealth, was still driving the decision-making processes of the leaders of all European nations leading up to World War One.
As for whether Social Darwinism (the repugnant philosophy introduced by Herbert Spencer) played a role in starting World War One, the evidence is more flimsy. Long before the term "survival of the fittest" was applied to a social context, the idea that stronger nation-states could and therefore should conquer weaker ones was taken for granted. The entire history of warfare is the history of the strong armies trampling over weaker ones. Also, the term Social Darwinism was almost always applied to domestic policy and interpersonal conflicts: ruthless businessmen used the theory to justify treating workers terribly and doing nothing to help the poor. Yet the term, Social Darwinism, wasn't often used to discuss foreign policy, partly because it wasn't needed, and also because political and military leaders had no patience for social theory.
Russia, Austria, Germany, France, England and Serbia each had great national pride. Add the fear of looking weak, losing military supremacy or territory to that mix and you have all the ingredients you needed for war.