Iran has been planning a counter-strike on the US for decades. Trump just lit the fuse.

• After the US killed Iranian military leader Qassem Soleimani in a rocket strike on Friday, Iranian officials swore vengeance.
• The attack has given Iran a reason to launch a retaliatory attack on the US.
• "They're going to respond, they have to as this is considered a declaration of war," a NATO military intelligence official told Insider.
• The official, based in the Middle East, said Iran has a huge array of potential targets because of broad US interests in the region.
• "They've been setting the table for decades for this moment," the official said.
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The US assassination of Qassem Soleimani, Iran's most powerful military and intelligence figure, is virtually guaranteed to draw an Iranian response somewhere in the region.

But precisely where is hard to predict because the Iranians have been planning for this moment for 40 years, according to a NATO military intelligence official based in the region.

"They're going to respond, they have to as this is considered a declaration of war," said the official who cannot be named because they work undercover in the Middle East.

"They've been setting the table for decades for this moment, their proxies have been carefully selected and trained in places like Lebanon, obviously, Bahrain, Iraq, Yemen and Gaza," they said.

"And the US has allies at risk in the case of Israel, but also its own bases all over Iraq, the Persian Gulf and even Afghanistan. Any of them can be targeted and usually by proxies that can serve as a layer of deniability."

The drone strike that killed Soleimani - as well as top Iraqi militia leader Abu Mahdi al-Muhandis - came after a rocket attack last week on an American base near Kirkuk killed a US contractor and wounded several others.

The US responded by striking camps in the Iraqi desert run by Kataib Hezbollah, an Iranian backed militia, killing more than 20 fighters.

The Iranian backed militias then responded by laying siege to the US embassy in Baghdad before a Reaper drone struck Soleimani's and Muhandi's vehicle just outside Baghdad's airport.

Iran immediately vowed to take revenge as a former commander of the Islamic Revolutionary Guard Corps called for a response to the killing.

"Martyr Lieutenant General Qassem Suleimani joined his martyred brothers, but we will take vigorous revenge on America," said former IRGC commander Mohsen Rezaei, who had worked with Soleimani in the past.

But where this revenge might take place is difficult to predict because of the number of US-linked targets that sit well within Iran's striking range, as demonstrated last September after Iran and its proxies launched 20 cruise missiles at a key Saudi oil facility, striking 19 out of 20 targets and knocking offline 5% of the world's oil supply.

Not only were the missiles not intercepted by the massive US military presence in the region, but the US has yet to officially determine where they came from.

"The September attack was a warning to the US and the Saudis that they could hit whatever they like in the region," said the NATO intelligence official.

"The Saudis learned that lesson, which is why you see them calming down in Yemen and easing aggression against Qatar. But did the Americans get the right message from that attack?

"I don't think so and its clear that US military and intelligence services are going crazy because they know there is an attack headed their way but the Iranians have so many choices of targets it's hard to predict where they will respond."

Iran's most powerful proxy force in the region, Hezbollah, is the gravest concern because of its impressive arsenal of guided and unguided rocket and missile systems pointed directly at Israel, which are capable of hitting targets anywhere in that country.

If the Iranians decide to respond by striking Israel, that situation could quickly spiral out of control as widespread missile and rocket attacks from Hezbollah positions in Lebanon or Syria could easily spark an Israeli invasion in response.

"If they ordered Hezbollah to strike Israel, that's your worst case scenario right there," said the NATO official.

"Second worst would be a successful strike on top American military targets in the Gulf, such as the naval station in Bahrain or the air base in Qatar. And both are within easy missile range of the Iranians."

Hezbollah, for its part, went on alert immediately after the news about Suleimani broke, according to a mid-level security commander in Beirut.

"We are mobilized now, with extra units in security positions around Beirut and in the south where all the missile and rocket teams have been put on standby," said the commander who does not have permission to speak with the media.

When asked if he thought Hezbollah would play a role in whatever happens next, he was both blunt and evasive.

"The Americans will pay for this murder of our leader," he added. "We are ready to hold anyone American or Israeli accountable for the blood of our martyrs. But we will select the time and place."