The fatal thing was the sentiment so well expressed in the last lines of In Flanders Fields, written in early 1915:

Take up our quarrel with the foe:
To you from failing hands we throw
   The torch; be yours to hold it high.
   If ye break faith with us who die
We shall not sleep, though poppies grow
         In Flanders fields.

It's the losing gamblers temptation, the feeling that if you don't carry on all that you have lost in lost in vain. But reinforced by the sense that failing to do so would be to betray those who have died. It repeats itself in every bloody conflict, and keeps them dragging on.