The Strait of Hormuz connects many of the Middle East's oil and commodity producers to the rest of the world. 

For Persian Gulf nations, their wares must make their way to the Arabian Sea via the narrow stretch of water.

And when Israel and the United States launched their military operation against the regime last month, Iran moved on it.

For decades, analysts and war gamers had warned that the closure of this passageway would be a likely consequence of a larger conflict with Iran. 

Many believed the regime would only move on the area as a last resort because closing the strait would affect Iran's own ability to send its oil out to sea.

But shortly after the war began, the radios of ships in and around the Persian Gulf crackled with a message from Iran's Revolutionary Guard (the IRGC): "From now on, all navigating through the Strait of Hormuz is forbidden."

Iran took the ultimate option, effectively closing the strait and unleashing pain on global markets, something Mr Trump cares about very much.

And some analysts believe it was last year's 12-day war that brought the Strait of Hormuz to this moment. 

"This particular body of water is one of the most important, strategic choke points in the world," said Farzin Nadimi, an Iran military analyst and senior fellow with the Washington Institute for Near East Policy.

He said when the 12-day war ended, there were internal ructions in the regime over the decision at the time to not threaten freedom of navigation through the Strait of Hormuz and for failing to put pressure on the world's energy markets.

Tehran suspected conflict would once again return to its doorstep, and when it did, the regime wanted to be ready.

"The thinkers within the Islamic establishment, they recommended that if there is a next time — and a good half of them actually believed that the next round would be coming at some point in the near future — pressure should be put on the world or oil markets," Dr Nadimi said.

"And, if the United States is involved, on the US companies involved in the Middle East oil and gas market and also on the Gulf countries because they house those air bases. 

The Strait of Hormuz lies between Iran to the north and Oman to the south.

Shipping in the strait is confined to a pair of 3.2-kilometre-wide lanes that connect the Persian Gulf to the Gulf of Oman and then the Arabian Sea. 

"Each country, according to international law and the law of the sea, has got about 12 nautical miles (approximately 22km) of territorial sea, which is part of the land of the coastal state," said international law expert at Flinders University Hossein Esmaili.

"That means that the narrowest part of the Strait of Hormuz is shared between Oman and Iran as their territorial waters.

"This means every vessel which passes through this strait, whether military vessels, ships, oil tankers ... they pass through either the Iranian territorial waters or Oman territorial waters." Dr Esmaili said under the law of the sea, while either nation might put regulations or checks in place, "they cannot hamper innocent passage of vessels, including military vessels".

That is how the strait is supposed to operate. But like so many aspects of this war, the Strait of Hormuz is now beyond the bounds of international law. 

As Israel unleashed its arsenal on the Iranian regime headquarters on the night the war began, it killed Ayatollah Ali Khamenei and put Tehran's long-held plans into action.

Many foreign ships now lie idle in the Strait of Hormuz, tucked into coves off Oman or inside the Persian Gulf, often uninsured to move and threatened with deadly attacks by Tehran if they tried. 

Those attacks have come in unconventional forms that traditional military methods struggle to eliminate, according to analysts.

Iran is effectively choking global trade, pulling from an arsenal that includes drones, anti-ship missiles and unmanned surface vessels.

By moving so much of the war into the Strait of Hormuz and by bringing the threat to its Gulf neighbours and the global economy, Tehran has hit its adversaries where it hurts the most.

Israeli Prime Minister Benjamin Netanyahu is absolute in his vision to see the regime destroyed, but Mr Trump is under immense pressure at both home and abroad over the economic pain caused by Iran holding the Strait of Hormuz. 

The United States military is the most mighty, but history shows that in asymmetrical warfare, the lesser power can win the day. 

The war comes to the Strait of Hormuz 

Prior to the war, the passage of ships through the Strait of Hormuz was presumed.

Over a 15-day period in early February, there were more than 2,000 ships that crossed the narrow passageway, but by the same period in early March, only 166 vessels made it through, according to tracking agency Marine Traffic.

Some vessels are making the crossing by deliberately turning their tracking transponders off, while others linked to China, India and Pakistan have passed with approval from Tehran.

Dr Nadimi has been following the ships that have been allowed to pass by the regime and noticed a change in the path taken by a Pakistan-flagged oil tanker on March 15.

"[Iran] no longer allows any ship to use the international shipping corridors in the Strait of Hormuz using Omani waters. They actually reroute them north of the Larek Island, completely inside Iranian waters, and then they're turned all the way back in toward the Gulf of Oman," he said.

"I think [the rerouting is] part of the psychology of this whole thing, they want to show that they have full control, and they want to prevent or dissuade or deter the Americans from interfering with this kind of controlling operation."

He called it a "selective closure". The regime has halted traffic in and out of the Persian Gulf, but allows friends, and its own tankers, to pass.

For some time now, Iranian leadership and military commanders have wanted to promote their role as "guardians of the entrance to the Gulf", according to Dr Nadimi. The war gives them an opportunity to do that more forcefully than ever before.

"They are making the transit too dangerous and, without actually mining the Strait, they [have] effectively stopped all shipping traffic in the Strait of Hormuz," Dr Nadimi said.

When US Energy Secretary Chris Wright was asked by a reporter if it was safe for shipping last weekend, he replied simply: "No. No, it is not."

Vessels that dare to venture through the strait without Iran's permission have been attacked in at least 20 confirmed incidents since March 1, according to shipping security monitors.

"What we're seeing is that this is becoming a linchpin in terms of the Iranian regime's strategy to try to bring pressure to bear on the Trump administration to hopefully withdraw from this military operation," said expert in international conflict at the University of New South Wales Jessica Genauer.

"Israeli Prime Minister Benjamin Netanyahu is a lot less concerned about the Strait of Hormuz. On the other hand, for President Trump … there is a lot of economic pain in the US right now directly from the closure of the Strait of Hormuz and the rise in oil prices, so President Trump wants to see the Strait of Hormuz opened as a first priority," Dr Genauer said.

US assets and allies have been targeted by the Iranian retaliatory strikes across the Gulf.  

Visualising Marine Traffic data shows how the Iranian regime is disrupting traffic through the Strait of Hormuz and the targeting of anything to do with America. 

Last week, in the north of the Persian Gulf, the US-owned Safesea Vishnu was preparing to sail to India when it was attacked by Iran, exploding into flames and forcing its crew to jump into the sea to escape the burning wreck. 

On March 11 UTC (March 12 Australian time), there were several clusters of ships on either side of the strait, including deep in the northern part of the Persian Gulf. 

This is now typical as hundreds of ships and their crews have dropped anchor, stranded as they wait for the war to abate.  

The Safesea Vishnu and the Greek-owned Zefyros were completing a transfer of 53,000 metric tonnes of a hydrocarbon mixture when they were attacked. 

Reuters reports two unmanned speedboats packed with explosives rammed into the Safesea Vishnu. 

The vessel drops off marine tracking, and then the Zefyros follows, before several vessels in and around the nearby Iraqi port respond to the scene.  

"What they're doing in the strait is, it sounds a very strange thing, but it's spoiling the war because it's spreading the costs not just in the region, but globally. And I would say they're having quite some success at that," said Neil Quilliam, an energy policy specialist and associate fellow at the Middle East North Africa program in Chatham House.

In the first statement issued since becoming Iran's new supreme leader, Mojtaba Khamenei​ identified the Strait of Hormuz as a key priority for the regime in its conflict with Israel and the United States.

He described Tehran's ability to block the body of water as leverage that "must definitely continue to be used".

Dr Quilliam said Iran wanted to "dig in and play this out".

"It's an old adage, but Iran is part of the geography, it's not going away. It's just going to stick there and bear this out," he said.

While few ships have a safe path through the Strait of Hormuz, Iran and its adversaries have also aimed their strikes at oil infrastructure along the Persian Gulf coast. 

On satellite imagery, the black smoke rising over the United Arab Emirates' Fujairah oil base and Iran's South Pars oil facility can be seen. 

The longer Iran has its foot on the neck of trade out of the Middle East, the more pressure on Mr Trump.

And, analysts say, with oil terminals now in play, the longer this goes, the more time it will take to course correct once it ends.

"I think that the strategy of the Iranian regime so far has been quite successful given that they know they don't have the military capability to really win in a direct military confrontation with the US," Dr Genauer said. 

"But they are certainly creating a pressure cooker of political costs stemming from those rising economic costs as a result of what's happening in the Strait of Hormuz." 

Iran's guerilla tactics

Control of the Strait of Hormuz is overseen by the powerful IRGC Navy, which reports directly to Iran's supreme leader, not the national navy that is responsible for controlling other areas of Iran's coastline.

The elite unit has access to a vast arsenal of rocket launchers, submarines and unmanned surface vessels, along with other unconventional arms that make it easy to target ships attempting to cross the Strait of Hormuz, and difficult to defend against them.

Surveillance of the strait is conducted from fortified island clusters controlled by the IRGC along the coastline, with Abu Musa, at the mouth of the Gulf, the most strategically significant.

Mountain ridges close to the coastline are used by the IRGC Navy as missile bases and other logistical staging areas because they are very well protected, Dr Nadimi said, while two types of tunnel networks hide its mosquito fleet of smaller, agile speedboats.

That system includes a raised tunnel and a wet tunnel, where the boats are "moved out on the water to offer a quicker reaction time," Dr Nadimi said.

"They look like … submarine dens in France in World War II, just smaller versions of those dens, and they use them to protect their submarines, their speedboats."

Dr Quilliam explained that because the coastline is very rocky and, in places, steep, there are lots of caves that the IRGC can exploit to hide or store boats.

"[They] can come out very quickly, target [an enemy vessel], and then head back in," he said.

Early in the war, the US targeted large ships in Iran's navy, with the military claiming it sank at least a dozen boats, including the Islamic Republic's "prize ship".

Dr Nadimi said the IRGC's arsenal includes a "great variety" of smaller boats, from several classes of heavily armed speedboats to submarines and fast-moving craft that are "very good at harassing shipping".

A mix of boats can work together to swarm a larger vessel, with their speed allowing them to close the distance rapidly and minimise the reaction time for defensive radars.

US officials have told media outlets they believe Tehran is also laying naval mines in the narrow passageway.

Iran is believed to have more than 5,000 of these mines in its arsenal, according to estimates by the Defense Intelligence Agency, which include moored, bottom and limpet mines.

Dr Nadimi explained the mines are triggered in three ways: through acoustic effects, with the mine programmed to trigger within a specific acoustic range, usually when it picks up the sound of a ship passing by, as well as a change of pressure, or a change in magnetic field, which can happen as ships with metal hulls pass by. 

The mines might not detonate, but the threat of them is enough to impact shipping.  

"The Iranians actually talked about that many years ago that they had mines sitting at the strategic waterways near the Strait of Hormuz, waiting to be detonated by remote control," Dr Nadimi said.

With the international focus on the Strait of Hormuz, the US began targeting IRGC Navy facilities along the coast this week.

On March 18, CENTCOM announced the US had deployed "multiple 5,000-pound deep penetrator munitions", otherwise known as bunker-buster bombs, on "missile sites along Iran's coastline near the Strait of Hormuz".

"There are mountain ridges very close to the coastline … [where] they have made their missile bases and other logistical staging areas in the mountains and they are very well protected, therefore the Americans have felt the need to use very heavy bunker busters, which can target those areas," Dr Nadimi said.

Some of IRGC's facilities have been struck by the US in several waves of its campaign, but Dr Nadimi said he expected "the speed boat capability of the IRGC" to remain.

"Just like their drones and their missiles, because they have so many of them and they have hidden them in so many caverns and tunnels," he added.

Why the lesser military power can win 

Pentagon officials told US politicians in a closed-door briefing on March 11 that America's war with Iran exceeded $US11.3 billion ($15.95 billion) in its first six days.

Part of that eye-watering amount was quickly spent in the early hours of the war when the US deployed some of the most expensive weapons in its arsenal.

It's not clear what Iran is spending on its defence and retaliation campaign, but in the battle for the Strait of Hormuz, the IRGC Navy appears to be using low-tech weapons to harass and attack multi-million-dollar ships.

Shahed drones can cost as little as $US20,000 to make, but to shoot them down, the US requires anti-missile interceptors — Patriots, THAADs — that can cost millions.

Working in tandem with IRGC speedboats are one-way attack drones like the Shahed-136 and Mohajer-6.

She said it only took "the threat of a couple of drones hitting that massive oil tanker … potentially [causing] a billion dollars' worth of damage" to impact shipping. 

Since the IRGC Navy cannot match the US militarily, it is relying on these unconventional methods and specialised weapons to try to exact economic costs on the US and the world in general, and so far it is working.

Dr Nadimi said this was classic asymmetric warfare. 

"[Iran] has for years created this capability, the fortifications, the hardware, large numbers of small boats — very fast, capable and heavily armed — [drones], and they had all these unmanned surface vehicles developed back in the 80s, and they perfected them with GPS and other means of guiding them," he said.  

In the strait, Iran is seeking to maximise its geographic advantage because it cannot defend itself in a conventional sense against the US and Israeli attacks.

"Basically, they have created all these capabilities to take on and watch a more powerful enemy, which is the United States, because they have been planning for this war from the very beginning," Dr Nadimi said. 

Iran is not the first to use guerilla tactics against a stronger opponent and the current war has drawn historical comparisons to other American wars, particularly in Iraq and Vietnam.

The lesson from history is that asymmetric warfare can draw a larger, financially equipped military into a quagmire that makes a quick victory extremely difficult to pull off, and perhaps most importantly, to announce. 

"The actor that has less military capability, by not losing, they essentially win or at least get to frame a military conflict as a win because they didn't lose against this actor with far greater military capability," Dr Genauer said. 

"What we do tend to see is that they will hang on, they will see it through, and they will have more staying power than the more powerful actor and will ultimately claim that as a victory." 

The question of where this goes next and if the next move from the US side involves significant escalation is one that looms large over Donald Trump.

Earlier this week, the US president was lampooning allies for not helping the US open the Strait of Hormuz to foreign shipping, before he changed his tune to say America didn't need any help.

"That's difficult because you've got a president with a very big ego here who will not want to back down. He's no longer in a position where he can simply claim victory, walk away, and the impact of this crisis will sort of diminish."

It's Dr Quilliam's view that the future of the war will depend, in part, on the risk appetites of each side.

"The Iranians are willing to sustain significant strikes against their infrastructure, against their leadership, against their people. Whereas typically, North American, European, Australians have much lower thresholds for pain, for fatalities, for injuries.

"That means that if the US were to deploy vessels to escort oil tankers out through the strait, they make themselves directly vulnerable.

"They can have all the firepower to hit whatever targets they want in Iran. But … if a US Navy vessel is hit, if it's struck, if there are major fatalities, then that is going to place pressure on Donald Trump back in the US, and that's where the sensitivity is for the country that has that tremendous force."

Credits: 

Reporting and digital production: Lucia Stein and Emily Clark

Satellite imagery and mapping: Mark Doman

Additional research: Cason Ho

Video: Shakira Wilson and Meredith Cate 

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