How did the assassination lead to world war 1?

Answer 1

Tensions between nations in European, including between Austria and Serbia, for a century after the Napoleonic Wars. The assassination was the last straw in Austria and Serbia's conflict and that conflict was the first one to cause the mobilization of troops. That mobilization caused Russia, then Germany and then France to go to war.

Answer 2

Actually, Franz Ferdinand's' death merely gave the excuse for war. The real reasons were that the Austrians and Russians were vying for control of the Balkan peninsula and Germany and Great Britain were in the middle of a naval arms race. The series of alliances also made it so that when Austria declared war on Serbia, Russia would aid Serbia, and when Germany declared war on Russia, France would come to Russia's' aid. Britain joined in because the Germans violated Belgian neutrality by invading France through Belgium ( the UK probably would have got involved even if Germany hadn't of gone through Belgium due to the entente cordial with France and the fact that Britain couldn't allow a single power to dominate Europe, as had been there policy for several centuries).