Answer:
The first world war was a global conflict because the strong alliances, treaties, and colonies often forced indirectly involved parties into battling each other.
For example:
After a Serb assassinated Archduke Franz Ferdinand of Austria while he was riding around in Bosnia, you, a New Zealander, must fight the Ottomans in Gallipoli because the UK declared war on Germany, in response to Germany's invasion of France and Russia.