1. Austria-Hungary- declared war on Serbia due to the assassination.
2. Serbia- became a participant by Austria-Hungary's war declaration.
3. Russia- allied to Serbia by their treaty
4. Germany- allied to Austria-Hungary by their treaty
5. France- allied to Russia
6. Britain- allied to France. Since Britain has entered the war, her many colonies have became participants, whether with direct military support or financial support.
7. Australia, Canada, India, New Zealand, and the Union of South Africa- entered the war because of their ties with Britain
8. Japan- entered the war due to its treaty with Britain.
As we can see, over 10 additional nations were involved because of their alliance treaties with other nations. Although some nations have entered the war with reasons of their own, it is undeniable that the alliance treaties were strong factors for the escalation of the war to a world-wide scale.
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Because
Each country had a secret alliance with maybe four or five other countries. so if one country declared war on another their allied countries would have to declare war as well.
Please explain what you mean by 'the English system'.
Leaders knew that if they declared war, their allies had to join the fight with them.
An alliance or agreement between two countries :)