What was the causes of the …

Some important causes of the First World War were:
Aggressive Nationalism – With the increase in colonial competition in the 19th century, the European powers became more and more aggressive in pursuance of their trade interests. Each power was determined to expand its territories and gain new colonies at the expense of the other. As a result, the political atmosphere in Europe became explosive.
Imperialism – Establishing an empire had become both a financial necessity and a status symbol for the European powers. Nations like England, France, Germany, Italy, Russia and Japan were competing with each other, both individually and in alliances, to establish ever larger empires across Asia and Africa.
Secret Treaties – Otto Von Bismarck, the German Chancellor, entered into treaties with Italy and Austria and created the ‘Trilateral Union’. This led to a spate of secret treaties. Russia signed an agreement with France. England entered into treaties with Russia, France and Japan. Such secret pacts created an atmosphere of suspicion and hostility in Europe.
Immediate Cause: The immediate cause of the First World War was the assassination of Austria’s prince Archduke Francis Ferdinand and his wife by an extremist Serbian nationalist in the city of Sarajevo. As a result, Austria issued an ultimatum to Serbia. When Serbia refused to cow down to pressure, Austria declared war on Serbia and thus began the First World War.