It is a cliché these days to observe that the United States now possesses a global empire—different from Britain's and Rome's but an empire nonetheless. It is time to move beyond a statement of the obvious. Our recent effort in Iraq, with its large-scale mobilization of troops and immense concentration of risk, is not indicative of how we will want to act in the future. So how should we operate on a tactical level to manage an unruly world? What are the rules and what are the tools?