Which factors led to the declaration of World War I?

Several factors led to the declaration of World War I. The immediate cause was the assassination of Franz Ferdinand, the heir to the Austro-Hungarian throne, by Serbian nationalist Gavrilo Princip on June 28, 1914. Therefore, nationalism, or a feeling of patriotism about one's own country or ethnic group, was one of the factors that led to the declaration of war. Serbian nationalist groups wanted to unite all Slavs in one country, an effort supported by Russia (a country that also had a number of Slavic people) and opposed by Austria-Hungary.
In addition, strong feelings of nationalism among the major European powers--Italy, Austria-Hungary, Germany, Britain, France, and Russia, led these powers to develop their militaries and to be very willing to use them to promote their national strength. Therefore, militarism was another factor that led to the war. Connected to the concepts of nationalism and militarism, imperialism was also a factor in the war, as the European countries had been involved in competing to establish overseas colonies for access to raw materials and markets. Therefore, tensions were running high among them in the decades before the outbreak of the war. 
As a result of competitive nationalist sentiment among the European countries, the nations were undergoing shifting balance of power agreements and new alliances. France, wary of growing German strength after the German unification of 1871 and the Franco-Prussian War, allied itself with Russia in the Franco-Russian Alliance. England also allied itself with Russia with the Anglo-Russian Entente of 1907, and the three countries eventually formed the Triple Entente. England was also wary of Germany because Germany's growing naval power threatened English naval power. Germany, Austria-Hungary, and Italy formed the Triple Alliance, and countries in those alliances promised to defend each other if they were attacked. Therefore, when the assassination of Franz Ferdinand occurred, Russia backed Serbia, and Austria-Hungary eventually declared war on Serbia. Austria-Hungary was given a "blank check" by Germany (this blank check meant that Germany provided unconditional support for Austria-Hungary). As a result, the major countries of Europe were all drawn into the war.