John Bounty Hound.
It was not about obscure politics.
UK would not have been involved if it was about Archdukes and the Balkans.

Duty meant something to the people of those times.
We were duty bound to defend Belgium when it was invaded.
We had a treaty.
People were outraged by the atrocities committed in Belgium and every reason to believe that we would be next..
They felt with justification they were fighting to defend Britain.
Everyone had a basic education and access to a free press.
They were not just fools too stupid to "understand when they told you the cause"