How did the Sarajevo Crisis in 1914 lead to the First World War?

Sarajevo Crisis, the immediate cause of the war was the murder of Archduke Francis Ferdinand, the heir to the Austrian throne, on June 28, 1914 by a Serbian at Sarajevo, capital of Bosnia. Austria served an ultimatum on Serbia on July 23 making eleven demands. Serbia accepted most of the demands except those that would have led to the loss of her sovereignty.

Austria declared war on Serbia on July 28, 1914. Russia started preparation for war to support Serbia. On August 1, 1914 Germany declared war on Russia and on August 3, war was declared on France. German troops marched into Belgium to press on to France on August 4 and on the same day Britain declared war on Germany. Thus, the First World War began.