The oil fields, pipelines and ports required a skilled workforce in order to operate. That workforce requires homes. They require stores to buy food and other supplies. Hospitals and doctors. Schools for their children. In other words, in order to operate the facilities critical to the United States, you need a workforce to do it -- and that workforce is gone. Unlike in other disasters, that workforce cannot return to the region because they have no place to live. New Orleans is gone, and the metropolitan area surrounding New Orleans is either gone or so badly damaged that it will not be inhabitable for a long time.

It is possible to jury-rig around this problem for a short time. But the fact is that those who have left the area have gone to live with relatives and friends. Those who had the ability to leave also had networks of relationships and resources to manage their exile. But those resources are not infinite -- and as it becomes apparent that these people will not be returning to New Orleans any time soon, they will be enrolling their children in new schools, finding new jobs, finding new accommodations. If they have any insurance money coming, they will collect it. If they have none, then -- whatever emotional connections they may have to their home -- their economic connection to it has been severed. In a very short time, these people will be making decisions that will start to reshape population and workforce patterns in the region.

A city is a complex and ongoing process - one that requires physical infrastructure to support the people who live in it and people to operate that physical infrastructure. We don't simply mean power plants or sewage treatment facilities, although they are critical. Someone has to be able to sell a bottle of milk or a new shirt. Someone has to be able to repair a car or do surgery. And the people who do those things, along with the infrastructure that supports them, are gone -- and they are not coming back anytime soon.

It is in this sense, then, that it seems almost as if a nuclear weapon went off in New Orleans. The people mostly have fled rather than died, but they are gone. Not all of the facilities are destroyed, but most are. It appears to us that New Orleans and its environs have passed the point of recoverability. The area  can  recover, to be sure, but only with the commitment of massive resources from outside -- and those resources would always be at risk to another Katrina.

The displacement of population is the crisis that New Orleans faces. It is also a national crisis, because the largest port in the United States cannot function without a city around it. The physical and business processes of a port cannot occur in a ghost town, and right now, that is what New Orleans is. It is not about the facilities, and it is not about the oil. It is about the loss of a city's population and the paralysis of the largest port in the United States.

Let's go back to the beginning. The United States historically has depended on the Mississippi and its tributaries for transport. Barges navigate the river. Ships go on the ocean. The barges must offload to the ships and vice versa. There must be a facility to empower this exchange. It is also the facility where goods are stored in transit. Without this port, the river can't be used. Protecting that port has been, from the time of the Louisiana Purchase, a fundamental national security issue for the United States.

Katrina has taken out the port -- not by destroying the facilities, but by rendering the area uninhabited and potentially uninhabitable. That means that even if the Mississippi remains navigable, the absence of a port near the mouth of the river makes the Mississippi enormously less useful than it was. For these reasons, the United States has lost not only its biggest port complex, but also the utility of its river transport system -- the foundation of the entire American transport system. There are some substitutes, but none with sufficient capacity to solve the problem.

It follows from this that the port will have to be revived and, one would assume, the city as well. The ports around New Orleans are located as far north as they can be and still be accessed by ocean-going vessels. The need for ships to be able to pass each other in the waterways, which narrow to the north, adds to the problem. Besides, the Highway 190 bridge in Baton Rouge blocks the river going north. New Orleans is where it is for a reason: The United States needs a city right there.

New Orleans is not optional for the United States' commercial infrastructure. It is a terrible place for a city to be located, but exactly the place where a city must exist. With that as a given, a city will return there because the alternatives are too devastating. The harvest is coming, and that means that the port will have to be opened soon. As in Iraq, premiums will be paid to people prepared to endure the hardships of working in New Orleans. But in the end, the city will return because it has to.

Geopolitics is the stuff of permanent geographical realities and the way they interact with political life. Geopolitics created New Orleans. Geopolitics caused American presidents to obsess over its safety. And geopolitics will force the city's resurrection, even if it is in the worst imaginable place.