Question

What single event set in motion the start of World War I?

Answer 1

The murder in the town of Serajevo (at that time in Serbia and today in Bosnia) of Archduke Franz Ferdinand of Austria (heir to the throne of the Austro-Hungarian Empire) by a serbian natioalist (Gavrilo Princip). This act set in motion a chain reaction of declarations of war between European nations.
Austria declared war on Serbia but Russia supported Serbia and came to help against Austria. Germany had to help Austria against Russia but France, worried about the German intervention, declared war so that Germany invaded Belgium and Holland (neutral) to bypass the Maginot Line (a chain of fortified stronghold on the border of France with Germany) to try to reach Paris. At this point England intervened to help France and declared war as well.