The Americans failed in which of the following capacity after occupying a territory, like that of Iraq?

Police

Undoubtedly, the US invasion of Iraq reveals several American vulnerabilities. The US has not been able to force the Iraqi people into submitting to the occupation forces of the US-led coalition. To fully understand the nature of American weakness, however, we need to have a historical perspective. Imperial powers through history have used military forces to accomplish only four tasks: to conquer, deter, punish and police. As the Iraq invasion shows, the American capacity to conquer is formidable. Similarly, the US capability to deter and to punish is self-evident. Where US military capability has thus far been shown to have serious weaknesses is in policing an occupied territory.