Whoever was behind the Washington plot was ready to start a war in the Middle East. The region is already on the brink of conflict over Iran's nuclear programme, with Israel increasingly twitchy over the progress Tehran is making towards a capacity to make nuclear weapons.

Leaked US State Department cables also make clear that the Saudi king, Abdullah, has repeatedly urged the US to "cut off the head of the snake" and attack Iran.

Against that backdrop, the assassination of the Saudi ambassador in Washington, with mass American casualties and perhaps an attack on the Israeli embassy too, would have ensured that the region went up in flames.

The US accuses the Quds Force (QF), the external operations wing of Iran's Revolutionary Guard, of being behind the plot. Given the hierarchy of the Iranian regime, such a huge undertaking would have required a direct order from the supreme leader, Ayatollah Ali Khamenei, who personally controls the QF.

Khamenei's involvement would be surprising, to say the least. Throughout his tenure – since the death of the Islamic republic's founder, Ayatollah Ruhollah Khomenei in 1989 – he has shown himself to be highly cautious and devoted to entrenching the power of the clerical regime.

Meir Javendafar, an Iranian-Israeli, said: "Khamenei's first priority is regime stability, and then a distant second, safeguarding the nuclear programme."

One speculative explanation circulating on Tuesday night was that Khamenei feels so threatened by internal opposition that he would provoke a foreign attack to allow himself to strengthen his grip on the country. But the opposition Green movement is currently in abeyance, and the nuclear programme is advancing steadily with little threat of concerted international action, or much global support for an Israeli strike.

The plot is also out of character for the QF. The unit is well-funded and has considerable freedom of action abroad. It is suspected of involvement in the bombing of a Jewish community centre in Buenos Aires in 1994, the funding and arming of Hezbollah in Lebanon, of Shia militias in Iraq, and even the Taliban in Afghanistan. In 2008, the head of the QF, Kassim Suleimani sent the US commander in Iraq, General David Petraeus a message in which he said, according to Petraeus, that he controlled Iranian foreign policy in the region.

However, to extend those operations to US territory would represent a significant leap in scope and ambitions. The way the plot was conducted would also suggest that the ruthlessly efficiently QF had lost its touch, being clumsy enough to transfer money from accounts under its control directly to US bank accounts.

Robert Baer, a former CIA agent with long experience of observing the QF, said: "This stinks to holy hell. The Quds Force are very good. They don't sit down with people they don't know and make a plot. They use proxies and they are professional about it. If Kassim Suleimani was coming after you or me, we would be dead. This is totally uncharacteristic of them."

Another possibility is that this is a rogue operation, perhaps organised by a faction inside the QF, without the Supreme Leader's blessing. There is an argument that it suited the purposes of President Mahmoud Ahmadinejad, who recently lost a bloodless power struggle with Khamenei.

If the attack succeeded, it would set in train events dramatic enough to turn the rigid, dusty hierarchy of the clerical republic on its head, giving Ahmadinejad the chance to seize the advantage.

Or the plotters could be fanatics inside the military establishment, bent on bringing the Revolutionary Guard to the top of the regime pyramid, beginning an open race to develop a nuclear weapon and confronting Israel directly.

"If this is a bunch of crazies, then anything is possible," Baer said.

All such possibilities are speculative. They would fundamentally reshape the Islamic Republic, and yet – for Iran experts – they are scarcely any more far-fetched that the idea that the Iranian establishment was behind a plot as brazen and reckless as this.

The thwarting of the plot almost certainly averted a conflict, but regional tension will escalate nevertheless. Any remote hope of resumed nuclear talks is dead for now. More sanctions and UN Security Council resolutions will be on the table instead.

Conceivably, that could break Khamenei's will to press on with the nuclear programme, and produce a compromise deal that defuses the threat of conflict.

Or it could just as plausibly convince him to accelerate the programme, persuaded that the regime's enemies are closing in. In that case, this extraordinary plot could yet succeed in sparking a new conflict in a very fragile region.