China prepares to head a great manufacturing empire. But empires unravel, usually from within. The forces that will determine which nations will dominate the 21st century may yet favor India’s emerging reach for global power status more than China’s determined grasp for that prize.Kamal Nath, India’s energetic minister of commerce and industry, states the case with economy: “China may win the sprint, but India will win the marathon.” In Nath’s view, this will be the Asian Century — but not in the ways many in the United States and Europe assume or fear.