Question

The assassination of Archduke Franz Ferdinand of Austria-Hungary at Sarajevo on 28 June 1914 set off the July Crisis. Austria-Hungary issued an ultimatum to Serbia; after Serbia’s partial rejection, Austria-Hungary declared war on 28 July 1914. A web of mobilizations and mutual defense obligations quickly expanded the conflict: Russia mobilized for Serbia, Germany declared war on Russia (1 August) and on France (3 August), Germany invaded neutral Belgium (4 August), and Britain declared war on Germany (4 August). These steps mark the outbreak of World War I in late July–early August 1914.
(make it more simple)

Answer 1

On 28 June 1914 a Serbian nationalist killed Archduke Franz Ferdinand in Sarajevo. That led Austria-Hungary to give Serbia an ultimatum; when Serbia did not accept all its terms, Austria-Hungary declared war on 28 July. Russia began to mobilize in support of Serbia, so Germany declared war on Russia (1 August) and then on France (3 August). Germany invaded neutral Belgium on 4 August, and Britain declared war on Germany that same day. These events quickly turned the crisis into World War I.