The world is hurtling into hyper-modernity, with science-driven innovation surprising even the technologically savvy. Events, from world politics to the global environment, seem thrown into general flux. But what postmodern armchair security strategist would ever expect to see the mid-ocean reappearance of something so antique as pirates? Pirates in the 21st century?

The idea that piracy could flourish in critical international waterways, two centuries after the Barbary coast powers were trounced by the U.S. navy, may seem preposterous. Naval power underwrote America’s debut as a global actor. America’s first grand strategist, Alfred Thayer Mahan, stressed global mobility and freedom of the seas. How could the United States—which remains the greatest naval power on earth—be thwarted, along with its allies, by piratical raiding parties of Somali fishermen in souped-up motor boats? The answer is astounding, and lays bare the West’s difficulties in irregular conflicts.

Each year, some 18,000 vessels travel between the Indian Ocean and the Mediterranean Sea via the Suez Canal, including oil tankers from the Middle East, container vessels laden with Chinese goods, and more humble craft whose hulls are filled with wheat and other commodities. They sail north along the Gulf of Aden and up the Red Sea to approach the Suez locks. As Mahan made clear with his straight talk about international straits—the “wormhole” routes between continents—the defense of key chokepoints in the worldwide delivery system is critical. Gibraltar, Hormuz and Malacca are strategic for navies and merchant vessels alike. So is the Suez Canal, as the man-made marvel of the 19th century, and its 20th-century cousin in Panama. Speed of transit is crucial in war as well as commerce, especially when margins are thin.

This system of shortcuts has now begun to break down, as the problem of failed states morphs into the spread of anarchic shipping lanes. The collapse of civil order in Somalia has imperiled the maritime route to the Suez Canal, leaving the global supply chain, shipping companies and sea-borne crews in a state of tumult. Over the past year, Somali gangs have motored in small skiffs from the shore into the Gulf of Aden bearing GPS locators, satellite phones, rocket propelled grenades and automatic weapons to attack more than 110 merchant ships. Somali marauders have captured 42 cargo vessels that could not outrun the pirates’ cigarette boat motors, seizing 815 sailors as hostages. They extorted as much as $30 million in ransom payments during 2008 and millions more in prior years, and continue to hold more than a dozen ships.

These pirates are not the lovable characters of stage and screen—neither Captain Hook outwitting Peter Pan nor the high-kicking musical Pirates of Penzance. This new species...