What one incident started WWI?

There isn't really ONE particular incident. It was a sort of gathering storm. Extreme nationalism contributed to it but the 'spark' as it were was when the Archduke of Austria was inated by a Serbian nationalist. Then it just kinda snowballed cause Austria-Hungry was obviously really mad and declared war on Serbia, who had an alliance with Russia so Russia declared war on Austria-Hungary but Austrai-Hungary was allined with Germany who had to declare war on Russia and Serbia. When Germany invaded Belgium, the French (allined to Belgium) had to declare war on Germany as well. Britian was ally of France as well so they declared war on Germany too.