What does “crossing our border” or “entering the United States” mean in 2014? After four decades of working the border in its many forms -- from my career in the Coast Guard, and going back to two years with the Drug Enforcement Administration in El Paso, Texas, in the 1970s -- I now believe we need to start by reimagining it.

The Internet, modern transportation systems, supply chains, climate change, and transnational groups from criminal syndicates to nongovernmental organizations all confound boundaries set down on a map. As a result, managing our borders in a way that balances security with commerce, enforcement with freedom of movement, and now the physical with the virtual world has become even more difficult. Unfortunately, much of the U.S. debate about border management still dwells on the southern border and the interdiction of undocumented immigrants and contraband.

That’s important, but the larger and more demanding task the U.S. faces is to build a border management system suited to the complexities of the 21st century. Here are some of the challenges that must be overcome:

A foreign-flagged vessel carrying containers from China to Los Angeles, one of several hundred a day that form a critical link in global supply chains, notifies the U.S. Coast Guard and Customs and Border Protection of its arrival 96 hours in advance. En route, a satellite-based system tracks the ship’s position, course and speed. The vessel’s history, its owner, cargo and other information are vetted against databases for anomalies. If necessary, the vessel is held offshore until it is boarded, or met by inspectors at the dock. Containers are subject to random inspections and detection technologies, but few are actually inspected. In most cases, the analysis of shipping information constitutes the virtual clearance of the cargo into the country -- a balance of security and trade that informs a new vision of the border.

Another example: A passenger boarding a flight to Boston from Dublin receives U.S. immigration, customs, agriculture and public health inspection before leaving Ireland, as do passengers on U.S.-bound flights from the Bahamas, Bermuda, Aruba and every major airport in Canada. Discussions are under way for a preclearance site at Abu Dhabi, a major air travel hub in the Middle East. In this case, the physical border is now extended to the port of departure, enabling government agencies to address potential threats outside the U.S. and speed the arrival of passengers once here.

Closer to home, even as illegal entries on the southern border decline and shift from Arizona to the Rio Grande Valley, other vulnerabilities get less attention. A case in point: More than 200,000 general aviation aircraft are registered in the U.S. -- from gliders to large transports -- and they access more than 10 times the number of airfields used by scheduled airlines. Although most are tracked, air-traffic-control radar and other sensors have a hard time following small aircraft over challenging terrains. Even more difficult are small vessels that are not required to carry locating devices in our coastal areas where there is little or no radar coverage.

The Internet presents a new kind of challenge. Its component infrastructures and technologies -- fiber-optic cable, switching systems, bandwidth and spectrum -- exist in every country. Yet to recognize a land border in cyberspace, let alone to “secure” it, is impossible. The Internet defies legal authorities based on land borders, creates ambiguity as to where events occur, and allows the transfer of money and the import-export of intellectual capital without the opportunity for traditional government inspection. It also represents an exposed “attack surface” for the transmission of malware, denial of service, criminal activity, identity theft and intrusion into industrial control systems.

At one level, dealing with such diverse “flows” of people, commerce and information in the global commons will entail lots of individual fixes: Recent regulatory initiatives by the Department of Homeland Security, for instance, will require advance screening of general aviation passengers similar to commercial airline travel. Now we need better visibility and information regarding the location, identity and intent of small vessels, including passenger data. We should improve technologies for nonintrusive inspections of containers, preferably at ports of debarkation. To better protect cyberspace, the U.S. needs to create a more effective means to pass information between the government and private sector where most of the cyber infrastructure resides. For starters, Congress must pass legislation that creates greater clarity on their respective roles.

Yet the more fundamental challenge is to knit together the activities of over two dozen component agencies and directorates currently involved in managing the border through the Department of Homeland Security. It also means consolidating information on people, cargo and conveyances that resides in separate stovepiped systems created before the advent of Homeland Security -- such as vehicle license plate reader data taken at the border, airborne surveillance video of maritime immigrant smuggling and the name of the freight forwarder of athletic shoes being shipped in containers from China to the foreign national being precleared in Abu Dhabi.

As the defining lines of the U.S. border become more blurred and the flows across it more complex, those in charge of managing it need to see the whole picture. That’s the only way they can balance security with economic growth and freedom of movement, and act efficiently and effectively. Whether detecting malware, interdicting ultralight aircraft, or inspecting passengers and cargoes far from home, a failure of imagination about our 21st century border is the last thing they can afford.

(Thad Allen, an executive vice president at Booz Allen, was commandant of the U.S. Coast Guard from 2006 to 2010.)

To contact the writer of this article: Thad Allen at allen_thad@bah.com.

To contact the editor responsible for this article: James Gibney at jgibney5@bloomberg.net.