What momentous historical event occurred on July 18 1914?

On July 18, 1914 the Serbian government refused to accede to the demands made by Austria in the so-called July ultimatum. This led to Austria declaring war on Serbia, wich in turn led to the start of WW I.

When the Austrians finally invaded Serbia, expecting an easy victory, they were soundly beaten by the Serbs and had to retreat hurriedly. There is a lesson there, if only I knew which one.