It was a covert line of attack described in Sun Tzu’s The Art of War. Your enemy expected the direct approach. He prepared for it and fiercely fought it off, resulting in severe casualties, the expenditure of major resources — and, ultimately, your own defeat. And yet, occasionally, there was another entrance, the fragile corridor. Your enemy never expected advancement via this route because it was labyrinthine and treacherous, and he often didn’t even know it was there. But if your army managed to make it through, it would deliver you not just behind your enemy’s lines but to his inner chamber, the heart of his heart