THE MEN who carried out last week's attack on the parade in Ahvaz, in Iran's southwestern province of Khuzestan, were well trained.
Four of them killed 25 people and wounded 70 others before they were shot dead.
The question is whether they were trained by Islamic State, or by the backers of the low-profile Ahvaz National Resistance, which also claimed credit for the attack.
IS is an independent, ultra-extremist Sunni Muslim movement that kills Shias (most Iranians are Shia) on principle, so there are no big political implications if it was IS that planned the attack.
If it was the Ahvaz National Resistance, however, then these were the opening shots in the next Gulf war, because the ANR is backed by Saudi Arabia and its smaller Arab allies like the United Arab Emirates and Bahrain.
Iran is convinced it was the latter.
"It is absolutely clear to us who committed this crime ... and whom they are linked to," Iranian President Hassan Rouhani said.
"The small Arab puppet countries in the region are backed by America, and the US is provoking them and giving them the necessary capabilities."
There is reason to suspect this is true.
The Arab countries of the Gulf are smaller and weaker than Iran, and have talked themselves into the paranoid conviction that Iran intends to destroy them, perhaps even to replace Sunni with Shia Islam. They talk of war with Iran as inevitable, and dream of drawing America into such a war to even up the odds.
President Donald Trump is also paranoid about Iran, and openly talks about overthrowing the Iranian regime.
But first there has to be a spark, some Iranian action that gives Trump and the Arab Gulf states a pretext for attacking Iran.
Saudi Arabia's de facto ruler, Crown Prince Mohammed bin Salman, said last year "we won't wait for the battle to be in Saudi Arabia. Instead, we will work so that the battle is for them in Iran".
If they get their war, both leaders expect the heavy lifting will be done by the US Air Force, but something bad has to happen on the ground first. Iran has to do something stupid.
How do you get it to do something stupid? Well, you could try supporting separatist movements in the ethnic minority areas that ring the country: Arabs in the southwest, Kurds in the northwest, Turkmen in the northeast and Baloch in the southeast.
With luck, the Iranian regime will over-react and massacre enough separatists (and innocent bystanders) to provide the pretext for an Arab-US attack.
After Saturday's attack in Ahvaz, Dr Abdulkhaleq Abdulla, a United Arab Emirates scholar who says what others don't dare, tweeted that the attack wasn't really a terrorist incident. He pointed out that "moving the battle to the Iranian side is a declared option",
and predicted the number of such attacks "will increase during the next phase".
If that's the Saudi/American strategy, then sooner or later they will goad the Iranian regime into committing some atrocity in return, and then we're away to the races.
It would be the fourth Gulf war in less than 40 years.
The first was the 1980-88 Iraq-Iran war, in which Iraqi dictator Saddam Hussein attacked the new revolutionary regime in Iran with the support of the US. Up to a million people were killed, most Iranian, but the only direct US support was to give Saddam's forces intelligence and targeting information for their attacks.
The second was the 1990-91 war between Iraq and most of its Arab neighbours, plus large numbers of American and other Western troops, after Saddam invaded Kuwait.
The third was in 2003, when President George W Bush invaded Iraq in the mistaken belief Saddam had links with the al-Qaeda terrorists who made the 9/11 attacks and/or was working on weapons of mass destruction.
The fourth, coming soon to a theatre a long distance from you, will be the US/Gulf Arab attack on Iran.
Of course, the attack in Ahvaz could just have been another meaningless spasm of hatred by Islamic State, and not a Saudi/American initiative at all.
But if not now, then soon ...
Gwynne Dyer's new book is 'Growing Pains: The Future of Democracy (and Work)'.