Harold Macmillan had it about right. Asked to identify the single most difficult aspect of his job, the long-ago British prime minister famously replied, "Events, dear boy. Events." Things happen. And they don't often help. To give another example of the same principle, it's a truism of military life that when a battle starts, the first thing you do is tear up the battle plan you made earlier. Not because the enemy doesn't do what 'Appendix XVII: Enemy Response' says he's going to do, bu...