Sunday's USA Today contained an article based on an interview of Michael Barbero, a U.S. Army lieutenant general who, until his retirement on Friday, commanded the Joint Improvised Explosive Device Defeat Organization. In the article, which bore the menacing headline "Military expert: Boston bombing 'not an anomaly,'" Barbero was quoted as saying the threat of improvised explosive devices will persist for decades.

In a time of shrinking military budgets, Barbero obviously hopes to save his organization, which many consider to be bloated and redundant. Nevertheless, what he said regarding the persistence of explosive devices was quite true -- and indeed, explosives have played a significant role in terrorism for hundreds of years. Because of this, Barbero's comments are a good opportunity to revisit the fundamentals of terrorism series I wrote last year and to discuss the role of improvised explosive devices in terrorism.

The Nature of Terrorism

At its heart, terrorism is a tactic used by the weak against the strong. In some cases it is used domestically by those who wish to change the regime but lack the power to do so by legitimate means or even by force. These actors use terrorism in an attempt to weaken the regime, highlight its vulnerability and recruit others to join their cause through what 19th-century anarchists first called the "propaganda of the deed."

Transnational terrorism works much the same way. It can be used to drive a foreign power out of a country, as Hezbollah's use of large vehicle bombs drove French and U.S. troops out of Lebanon in the early 1980s. It can also be a vindictive tool to exact revenge, as were the Libyan attacks against U.S. interests in the 1980s. Finally, international terrorism can be used to highlight the vulnerability of a super power in an attempt to recruit more fighters or, in the case of al Qaeda and the series of attacks that included 9/11, spark a global uprising.

Because of the nature of terrorism, the people who conduct terrorist attacks generally lack the sophisticated weaponry associated with the regime or foreign power they oppose. This means that in most circumstances those wishing to conduct a terrorist attack must utilize weapons that they have readily at hand or can fabricate themselves. Terrorist planners can be greatly aided in cases where they have state sponsorship and where a friendly government can provide them with sophisticated, reliable weaponry. The efforts of many of the Palestinian terrorist groups, such as Black September and the Abu Nidal Organization, were greatly aided by the patronage of states, as were the efforts of European Marxist groups like the German Red Army Faction and the Italian Red Brigades.

Improvised Explosive Devices

Improvised explosive devices have always been one of the primary tools of terrorism. As far back as 1605, Guy Fawkes plotted to destroy the British Parliament using barrels of gunpowder. Bombs also played a significant role in anarchist terrorism of the late 19th century. Explosive devices are important to terrorism not only because they are effective at producing death, carnage and mayhem but also because of the powerful impact such theatrical attacks have on human imagination. Bombs can generate an incredible sense of panic, anxiety and the terror that gives terrorism its name.

In terms of "bang for the buck," explosives are a very cheap and effective way of sowing terror. While potential terrorists remain interested in chemical,biological and even radiological weapons, for the foreseeable future explosives will continue to be a more efficient and effective way of killing people.

A comparison of the 1995 Aum Shinrikyo attacks against the Tokyo subway system and the jihadist attacks against commuter rail in Madrid in 2004 and the July 2005 London subway and bus bombing illustrates the difference in cost and effectiveness. Aum Shinrikyo spent millions of dollars to construct its biological and chemical weapons program, and the most deadly of its many attacks killed only 12 people. By comparison, the Madrid attack cost the perpetrators thousands of dollars and resulted in 191 deaths, and the London attacks cost hundreds of dollars and killed 52 plus the suicide bombers.

Bombmaking instructions are also easy to find -- and were even before guidelines and tutorial videos existed on the Internet. Information on how to create improvised explosive mixtures and devices has been disseminated as long as there have been radicals and explosives. Even in relatively modern times, publications such as The Anarchist Cookbook and The Poor Man's James Bond predate the Internet.

It is important to remember that the word "improvised" does not necessarily mean poorly constructed or inferior. Improvised explosive devices are merely improvised in the sense that they are not factory-built munitions. Perhaps the best way to understand them is to think of them as improvised in the sense of a jazz solo -- and in fact, many improvised explosive devices are masterful works of destructive art. However, the quality of an improvised explosive device is dependent upon the skill of the bombmaker, and skill varies markedly. It can range from Miles Davis-like virtuosity at the high end to something akin to a child with a toy trumpet at the low end.

The list of explosive virtuosos would include Abu Ibrahim, who was named the "grandfather of all bombmakers" for his innovative improvised explosive device designs during his time with Black September, the Popular Front for the Liberation of Palestine and his own group, the 15 May Organization. Ibrahim was known for creating sophisticated devices that used plastic explosives and a type of electronic timer called an "e-cell" that could be set for an extended delay.

Another Palestinian bombmaker, Yahya Ayyash of Hamas' Izz al-Deen al-Qassam Brigades, became famous for his ability to craft improvised explosive mixtures, such as triacetone triperoxide for rocket warheads and suicide devices, even after the Israelis began to restrict materials allowed into the Palestinian territories. The Israelis eventually assassinated Ayyash due to the danger he posed.

The Exceptional Bombmaker

When operating against robust security and in a hostile environment, innovation and imagination become critical traits for a bombmaker to be successful. Since the beginning of terrorism, there has been a constant arms race between terrorist planners and security forces. Every time security is changed to adapt to a particular threat, the terrorist planner must come up with a new attack plan (often involving a new type of improvised explosive device) to defeat the enhanced security measures. For example, terrorist planners, such as Imad Mughniyeh's team in Lebanon, adjusted to increased perimeter security at embassies and government buildings by developing and deploying very large vehicle-borne improvised explosive devices. Jihadist planners have responded to changes in airline security measures by adopting baby doll bombs, shoe bombs, liquid bombs and underwear bombs.

Not only can improvised explosive devices come in a number of different shapes, they can also be designed to serve different functions. When Provisional Irish Republican Army planners wanted mortars to conduct standoff attacks, their bombmakers were able to design improvised mortars. A February 1991 attack on 10 Downing Street in London using such devices nearly succeeded in killing then-Prime Minister Margaret Thatcher and her War Cabinet. The Japanese Red Army learned how to make improvised mortars from Irish trainers in Libya and used them in attacks against U.S. embassies in Jakarta, Rome and Madrid. Faced with a lack of anti-tank guided missiles or anti-tank rockets to attack armored military vehicles or armored sedans carrying important individuals, terrorist bombmakers have developed devices that employ explosively formed penetrators and platter charges to punch through armor.

It takes a unique, imaginative individual to keep advancing the art of bombmaking. A master bombmaker might be able to show a pupil how to build a simple improvised explosive device or maybe even something specialized like a shoe bomb. The pupil may even become quite proficient at assembling such devices. But unless the pupil is innovative and imaginative, he will not be able to go beyond his training to invent and perfect the next technology needed to stay ahead of security countermeasures.

This was illustrated quite clearly in the case of would-be Times Square bomber Faisal Shahzad. Shahzad was trained in bombmaking tradecraft at a camp in Pakistan, but when he was forced by circumstances to construct a device with components that were quite different from those he had been provided in his training, he failed to construct a viable device.

There is generally a learning curve for bombmakers. Progress can be accelerated somewhat with formal training, but there is no substitute for practical experience and trial and error. Typically, a bombmaker's devices will grow more deadly over time in subsequent operations as a result of this process.

That said, there is a big difference between a technician and an inventor -- a difference perhaps best illustrated by a return to our jazz analogy. A student can learn to play the saxophone and perhaps even to mimic a jazz recording note for note. But it is quite another thing for that student to develop the ability to improvise a masterful solo on the fly like saxophonist John Coltrane could.

In music, Coltranes are rare, and so it is with exceptional bombmakers -- masters of destruction who can create imaginative and original improvised explosive devices capable of defeating security measures. But master bombmakers are not the only threat. As the Boston attacks showed, even novices can construct and deploy basic devices that can be quite deadly. The fact is that improvised explosive devices, both sophisticated and rudimentary, will indeed continue to pose a threat as long as terrorism exists.