When White House officials first heard an informant’s report last spring of an Iranian plot to kill the Saudi ambassador to Washington, they found it implausible. They asked the same question we all have been puzzling over since the indictment Tuesday of the alleged plotters:

If the Iranians planned such a sensitive operation, why would they delegate the job to Mansour Arbabsiar, an Iranian American former used-car dealer, and a hit team drawn from a Mexican drug cartel? To say it sounded like a spy novel is unfair to the genre. The wacky plot was closer to that of an Elmore Leonard “caper” novel, along the lines of “Get Shorty.”

But over months, officials at the White House and the Justice Department became convinced the plan was real. One big reason is that the CIA and other intelligence agencies gathered information corraborating the informant’s juicy allegations — and showing that the plot had support from the top leadership of the elite Quds Force of the Iranian Revolutionary Guard Corps, the covert-action arm of the Iranian government.

It was this intelligence collected in Iran — not tips from someone inside the Mexican drug mafia — that led the Treasury Department to impose sanctions Tuesday on four senior members of the Quds Force who allegedly were “connected” to a plot to murder the Saudi ambassador. The alleged conspirators included Qassem Soleimani, head of the Quds Force, and three deputies who allegedly “coordinated” the scheme.

Let’s make two assumptions: The first is that the allegations made by the prosecutors about Arbabsiar are true. This seems likely, given that he’s a cooperating witness. The second is that Quds Force operatives were willing to talk with Arbabsiar about a covert operation in the United States. That, again, seems pretty clear from the transcript of the Oct. 4 telephone call Arbabsiar made to his main Quds Force contact, Gholam Shakuri, under prosecutors’ direction.

The puzzle is why the Iranians would undertake such a risky operation, and with such embarrassingly poor tradecraft. Soleimani and his group are some of the savviest clandestine operators in the world. In past columns, I’ve likened him to “Karla,” the diabolically clever Russian spymaster in John le Carre’s novels. Why would the Iranian Karla turn to such a bunch of screwballs?

Here’s the answer offered by senior U.S. officials: The Iranians are stressed, at home and abroad, in ways that are leading them to engage in riskier behavior.

Officials say Quds Force operations have been more aggressive in several theaters: in Syria, where the Iranian operatives are working covertly to help protect the embattled regime of President Bashar al-Assad; in Iraq, where the Quds Force this year stepped up attacks against departing U.S. forces; in Afghanistan, where they have been arming the Taliban; in Azerbaijan, where they have been more aggressive in projecting Iranian influence; and in Bahrain, where their operatives worked to support and manipulate last spring’s uprising against the Khalifa government. (Shakuri, who was indicted Tuesday, is said to have helped plan Quds Force operations in Bahrain.)

But why the use of Mexican drug cartels? U.S. officials say that isn’t as implausible as it sounds. The Iranians don’t have the infrastructure to operate smoothly in the United States. They would want to use proxies, and ones that would give them “deniability.”

“They’re very willing to use all kinds of proxies to achieve specific clandestine foreign-policy goals,” says a senior U.S. official who has been briefed on intelligence reports.

It would mark a significant escalation for Iran to conduct terror operations inside the United States. But such attacks would come against the background of a secret war in the shadows that began in 1983, when the predecessor to the Quds Force recruited Lebanese Shiite bombers to destroy the U.S. Embassy and Marine barracks in Beirut, killing more than 300. The organization was then known internally (by the few who knew of it) simply as “Birun Marzi,” or “outside borders.” Then it took the cover name “Department 9000,” and later, in deference to the Arabic name for Jerusalem, Quds Force.

A final factor in this unlikely plot is the political turmoil in Tehran. The Quds Force is seen by analysts as the executive-action arm of Ayatollah Ali Khamenei, Iran’s supreme leader, who is in a bitter battle with President Mahmoud Ahmadinejad. During this feud, the Iranian ministries of foreign affairs and intelligence have increasingly been hobbled, leaving the field to the Quds Force. It’s a chaotic situation tailor-made for risk-takers, score-settlers and freelancers.