How did alliances lead to the outbreak of WWI

They did not. The assasination of Arch Duke Ferdinand did.

The Alliance system was one of the causes of World War I. Prior to the war there were two alliances. One was the Triple Alliance. This included Austria-Hungary, Germany, and Italy. The other alliance was the Triple Entente. This included Soviet Union (Russia), Great Britain, and France. The danger of these alliances is that if a member from one alliance declared war on a member from the other alliance, the conflict would quickly escalate. That is what happened in World War I.

When the Austrian archduke Franz Ferdinand, who was the next king of Austria-Hungary and his wife Sophie was assassinated by Gavrilo Princip, a Serbian nationalist in Sarajevo on 28 June 1914 which set of a chain of events which led to outbreak of the WW I. Austria-Hungary made a series of demands on the government of Serbia. When the government couldn’t meet all of these demands, Austria-Hungary, with the support and approval of Germany, declared war on Serbia. Austria-Hungary made a series of demands on the government of Serbia. When the government couldn’t meet all of these demands, Austria-Hungary, with the support and approval of Germany, declared war on Serbia. Austria-Hungary made a series of demands on the government of Serbia. When the government couldn’t meet all of these demands, Austria-Hungary, with the support and approval of Germany, declared war on Serbia. Since Serbia and Russia were very close friends and allies, Russia declared war on Austria-Hungary. Germany then declared war on Russia. France and Germany then declared war on each other. Eventually, Britain declared war on Germany when Germany invaded the neutral country of Belgium. Thus, what was originally a conflict between Austria-Hungary and Serbia quickly escalated into World War I, involving many countries throughout the world.