How did the alliances between European countries lead to WWI?

I would argue that it is more accurate to say that the system of alliances caused WWI to become a major war once it started than to say that the alliances actually caused the war to begin.  Let us examine why this is.

The actual onset of the war was caused by the assassination of Archduke Franz Ferdinand and the Austrian ultimatum to Serbia that followed it.  When the Archduke was assassinated, Austria-Hungary was enraged...

I would argue that it is more accurate to say that the system of alliances caused WWI to become a major war once it started than to say that the alliances actually caused the war to begin.  Let us examine why this is.

The actual onset of the war was caused by the assassination of Archduke Franz Ferdinand and the Austrian ultimatum to Serbia that followed it.  When the Archduke was assassinated, Austria-Hungary was enraged and blamed Serbia for helping to stir up Slavic nationalism in the Balkans.  Therefore, they issued an ultimatum that Serbia could not possibly have complied with.  This started the war without any help from the system of alliances.

However, the war might have remained as a war between Austria and Serbia had it not been for the alliances.  The Russians entered because they were allied with the Serbs.  Then the Germans entered because they were allied with the Austrians.  Then France entered because of their treaty with Russia and Great Britain entered because of its treaty with France.  This, of course, brought all of Britain’s dominions into the war.  Thus, the system of alliances caused this war to become a world war.  What might have been a war between two countries became a general war that ended up affecting countries all over the world.