Britain was treaty bound to defend Belgium.
Belgium was invades by a ruthless aggressor and suffered atrocities and massacres of its ordinary people and children.
They were headed our way.
We sent the army we had.
Yes it was hopelessly outnumbered and forced to fight a desperate fighting retreat, but the invaders were stopped.
It was after that retreat when all hopes of a short war were dashed and defeat a real possibility that there was the greatest surge of volunteers.

They understood the cause quite well.
It was not in vain.