For want of a nail the shoe was lost.
For want of a shoe the horse was lost.
For want of a horse the rider was lost.
For want of a rider the battle was lost.
For want of a battle the kingdom was lost.
And all for the want of a horseshoe nail.

Holmes repeated the proverb to himself—the proverb that had he had memorized as a young boy, years before he realized its practical application by combining its guiding principle with his unique powers of observation. Certainly it applied to Professor Moriarty. In his conceit Moriarty had overlooked the very traces that led Holmes to him. Even after Holmes had uncovered his identity, Moriarty's kingdom seemed impenetrable. But one fateful day he made another false step and exposed his kingdom to destruction. Yes, Moriarty did want a nail, but it was not a horseshoe nail. Holmes had stolen it from him, and in his hands it was a coffin nail.

And yet…

Was Holmes not in the same position? His confidence that Moriarty would ambush him, his lack of observation—these created the want of a nail that lost him Watson—the rider? The battle? No, he would not become enamored of this conceit! Focus on the facts! Still, such a small mistake—he had momentarily forgotten the mail carriage's route, nothing more—and now Watson was in mortal peril.

Yes, it was a blow to his conceit—from the loss of a nail to the loss of a battle with no intervening steps. All that Holmes could do now was see to it that this not be the war's deciding battle.