*Clears throat*
Serbia rebels kill Austria prince
Austria wants Serbia to pay for what their rebels did.
Serbia can't pay what Austria is asking.
Austria declares war.
Russia has a protection pact with Serbia. Russia declares war on Austria.
Germany has a similar pact with Austria. Germany declares war on Russia.
France has a similar pact with Russia. France declares war on France.
Germany invades france through the Netherlands.
The Netherlands have a protection pact with the UK. The UK declares war on Germany.
Overall this is a strictly European war. Only problem is that they have colonies on every continent making it a world war.