How did the assassination of the Austrian archduke and his wife in Sarajevo lead to world war 1?

The Archduke Ferdinand was heir to the Prussian throne. His death caused an outage in Prussia which declared war on Serbia, who in turn had treaties with other European countries to aid it in case of war. As each country was brought into war, it would have treaties with another that would oblige it to aid it in case of war. It was the same for both sides. It was very much like falling dominoes. The assassination of the Archduke is what tipped the the first domino.