How did the alliances in Europe help to bring about the First World War?

The alliance systems basically ensured that what began as a local conflict in the Balkans became a continent-wide conflict. When Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire, was killed by a Serbian assassin in Sarajevo, the Austrians, with German support, held the entire nation of Serbia directly responsible for the act. They issued an ultimatum that the Serbians could not reasonably accept, and declared war on them. When they declared war, Russia, a Serbian ally, mobilized its armed forces against Austria-Hungary. Germany then declared war on Russia. At this point, Russia's allies France declared war on Germany. When Germany violated Belgian neutrality, Great Britain, a Belgian ally, declared war on the Germans. So all of Europe was dragged into what began as a conflict over Serbian grievances against Austria-Hungary. The fact that each of the belligerent nations held imperial possessions in Africa and elsewhere ensured that it spread around the world as well.