What incident triggered World War 1?

Historically, it is seen as the assassination of Arch Duke Ferdinand, the heir to the Austro-Hungarian throne. He was shot in July 1914 by a Serbian nationalist group "The Black Hand". Austria-Hungary declared war on Serbia, which prompted Russia to side with Serbia, France sided with Russia, Germany declared war on France, and invaded through Belgium which caused Britain to declare war on Germany... Very complicated. However, there was huge tension in Europe at that time as the great empires were shifting and changing. It is now known that the Serbian nationalist group was sponsored by Russia. It is probably the case that the assassination was simply the excuse Austria-Hungary was looking for to gain new territory, and they unwittingly caused a cascade of declarations of war, based on tensions between empires and historic alliances, that lead to WW1.