That is to say, you can't have it both ways. It's not a matter of saying "They could have achieved their aims in other less murderous ways." They couldn't. The only way for Asquith to avoid war would have been, essentially, to take the decision to lose it. That would have been a very courageous decision, in the Sir Humphrey sense.
The same with Iraq. The issue isn't "Is the world better off without Saddam?" It's whether the good of having no Saddam is worth the evil of not having the what, half million to a million Iraqis who died along the way.
Which does, to some extent, underline that the person who starts it - who walks across someone else's border carrying a rifle - bears a heavy responsibility. Then, Bethmann-Hollweg and Berchtold. Now, George Bush.
Placing Obama in the position of Lloyd George or Clemenceau - someone who inherited it but hasn't got the authority to stop it and take the loss and just has to double down until something occurs that lets someone claim victory.