This year has seen a spike in such encounters. Western ships in the Gulf are now regularly shadowed by the smaller crafts of the Iranians. When U.S. strategists make lists of the many challenges posed by Iran, the capabilities of the IRGCN, as it is known, quickly rise to the top. The Gulf, of course, is indispensable to the smooth flow of energy resources (in 2009, more than 15 percent of oil traded worldwide moved through the Strait of Hormuz, the chokepoint between the Gulf and the Arabian Sea), and the Iranians are well aware of their ability to strangle the global economy.

Only Iran's nuclear program -- the one its leaders claim is entirely peaceful in nature even as they develop the technology to make triggers for nuclear weapons -- is a greater preoccupation.

The U.S. government, and the media, is distracted at the moment by the alleged plot by the Quds Force, a unit of the Revolutionary Guards, to assassinate the Saudi ambassador to the U.S. I'm ambivalent about this extraordinary allegation. On the one hand, it seems improbable that the Iranians, who are very good at committing dastardly acts (witness the devastating bombings of the Israeli Embassy and a Jewish center in Buenos Aires in the 1990s) would subcontract such a sensitive mission to a Mexican drug gang, as the U.S. has alleged. It also seems improbable that the Iranians would risk outright war by conducting such an assassination on U.S. soil.

On the other hand, the attorney general, Eric H. Holder, and the FBI director, Robert Mueller, aren't the type of men who would make this sort of accusation lightly. And, perhaps more importantly, Iran isn't an entirely coherent place at the moment: The order to launch such a reckless operation could have come from any corner of Iran's chaotic government, or it may have been entirely self-initiated by a small band of radicals eager to bring about a confrontation with the U.S.

Which brings us to the particular challenge posed by the navy of the Revolutionary Guard: It could very well spark a war with the West without an express order from Tehran.

In a new report issued by the Institute for the Study of War, Navy Commander Joshua Himes, an expert on Iranian naval forces, suggests that this is a highly credible scenario: "Though Iran could make a rational decision to initiate a limited kinetic strike to further its strategic aims, an alternative scenario exists," he writes. "An incident could arise from having the less professional (or more fervent) IRGCN sailors overstep their commanders' intent, miscalculate at a tactical level, and set off a chain of events that could spiral into conflict."

And if such an incident happens, there is no easy way for the U.S. to de-escalate it: Iran recently rejected an American offer to establish a hot line between the two countries' militaries.