Answer:
Start of a local conflict:
Archduke Ferdinand of Austria-Hungary was assassinated by Gavrillo Princip of the Serbian terrorist group, "the Black Hand". In response, Austria-Hungary gave Serbia an ultimatum to Serbia, threatening war if it was not accepted. Serbia refused the condition that would allow Austria-Hungarian officials/agents into Serbia to go after the Black Hand. A-H considered this a refusal of the ultimatum and declared war on Serbia.
Spread to a World War:
July 28: Austria-Hungary declares war on Serbia.
July 29: Russia, an ally of Serbia, joins the war against Austria-Hungary.
July 30: Germany, a Triple Alliance member (along with A-H and Italy), joins war against A-H. Germany and A-H declare war on Russia.
August 1: France declares war on Germany.
August 4: Britain declares war on Germany due to German threat on Belgian sovereignty/neutrality.
The Ottoman Empire joined on the side of the Triple Alliance.
All British dominions (Canada, Australia, etc.) automatically declared war on Germany upon the British declaration.