Live updates: Follow the latest news on US-Iran war
With US troops arriving in the war zone in force and the Strait of Hormuz blocked, the Pentagon has one unused option to break the deadlock.
It is an operation led by US Marines and the 82nd Airborne Division to capture Kharg Island in the Arabian Gulf.
The Marines have arrived in the Centcom operations sphere aboard the USS Tripoli with its accompanying amphibious ships. CBS News said on Monday that hundreds of US Special Forces personnel, thought to included Navy Seals and Army Rangers, were on site as well.
President Donald Trump told the Financial Times at the weekend that Washington could take control of Iran's oil by occupying Kharg island. On Truth Social on Monday, he threatened the "obliteration" of the island.
“Taking an island like that is not going to be a huge problem, but I don't think it'd be bloodless for the US,” said Nick Reynolds, a land warfare specialist at the Royal United Services Institute think tank in London.
The option for capturing Kharg Island, which hosts 90 per cent of all Iran's oil exports, offers Washington the hope that it would force Tehran to accede to its demands.
The Pentagon has confirmed that its A-10 Warthog ground attack aircraft and Apache helicopters are flying missions against Iranian ground forces in and around the island.
This could help “soften up” remaining Iranian positions before the landing and reduce any perceived threat from Iranian boats, mines and cruise missiles.
Experts said US commanders would ideally launch an amphibious assault using hovercraft and armoured vehicles. After landing, the Marines could take out any remaining Iranian positions with the support of attack helicopters and fighter jets. Traps could easily lurk and Iran is likely to continue to contest the occupation.
If the 22-square-kilometre island’s defences have been “adequately suppressed”, as the US has suggested, then it should be “a comparatively trivial task to successfully land a force”, said Dr Lynette Nusbacher.
“The question then would be how long are they going to remain in place and how do they do sustainment operations.”
But she said that “Iranians would certainly have all of Kharg Island”, which is 25km off the mainland, “registered for artillery, especially rocket artillery”.
Marine marauders
With a large helicopter capability, which includes Ospreys that carry 32 troops and Sea Stallions that can carry 55, the US has options beyond seizing territory.
“There is the possibility of raiding into Iranian territory proper in an attempt to stop some of those [longer-range] strikes on shipping with direct action,” says Mr Reynolds.
They could also be used in helicopter raids against the islands from where Iran has been launching fast boats capable of mining the strait or attacking ships.
But there are reasons to be wary. The US will want to avoid a repeat of the disastrous 1980 Eagle Claw operation to rescue American embassy hostages. It resulted in helicopter crashes and the loss of eight personnel.
That will surely be considered when Pentagon chiefs decide what steps to take next as the war with Iran intensifies.
Opening the Strait of Hormuz is ultimately regarded as crucial to prevent a deteriorating economic situation from getting worse.
Military experts believe that achieving such an objective would require a major land, sea and air force deployment in the area to eradicate the threat posed by Iran by commercial ships.
Aerial attacks on Iranian positions are continuing, using helicopters and fighter jets, but more needs to be done. By now, Pentagon planners will have presented options to US President Donald Trump. Among them will be a land attack that could force Iran to relinquish its strangulation of the strait.
A specialist rapid response unit of 2,200 marines, the 31st Marine Expeditionary Unit, is reported to be on its way to the Gulf aboard the amphibious assault ship USS Tripoli.
It has been suggested that they could be used, along with attack and transport helicopters, to secure the strait to enable all ships to pass through. They will not be in place for such an operation until about April 1, however, and pulling it off will be far from straightforward.
They could be joined by the paratroopers of the 82nd Airborne Division, the army’s immediate response force that can deploy within 18 hours of notification.
The potential operations for the 31st Marine Expeditionary Unit, are varied and spread across Iran.
The marines could be used for commando raids on the Iranian Hormuz littoral, taking out threats to shipping such as missile batteries or mine stockpiles that are difficult to hit from the air.
Target board
“The highest priority for the Americans is to open up the strait because the Iranians are winning this war right now and the only way to stop that is to get the tankers moving,” said Dr Nusbacher, a former military intelligence officer.
Former US Marine veteran John Hackett agrees. “Only a handful of ships have transited, which indicates there's almost 100 per cent control of the straits by Iran and zero per cent by the United States,” he said.
Take the strait
“Somebody right now is certainly looking at a target pack and asking: ‘Can we open the strait by using land forces?’” said Dr Nusbacher. “Someone has also clearly briefed that they could be doing a lot of raiding, and USS Tripoli could be optimised for aviation with loads of helicopters on board.”
Key to unravelling the deadlock will be securing the Iranian side of the Strait of Hormuz. Given its mountainous nature, that would be a difficult task for ground troops.
The USS Tripoli is carrying a force of marines complete with their heavy lift Sea Stallion helicopters, Viper attack helicopters and Osprey aircraft.
A “helicopter heavy” force gives the marines the ability to carry out an air assault on targets that have already been hit from the air by US and Israeli fighter jets.
A marine unit has its own weaponry to fight autonomously, from the eight-wheeled LAV-25s, which mount a 25mm cannon, to 155mm artillery, mortars and anti-tank missiles.
Littoral raids
It has been suggested that they could land on Iranian ground at Bandar-e-Jask, on the Gulf of Oman side, then move steadily northwards along the coastline that controls the strait towards Bandar Abbas, taking out Iranian positions.
“The force could then work its way up the Iranian side of the strait, clearing as it goes, but you would need a lot more than a brigade there,” said Jeremy Binnie, an expert at defence intelligence company Janes.
Mr Reynolds, at the Rusi think tank, believes helicopter-borne marines are “capable of conducting a lot of raids to deny the shores of the Strait of Hormuz to Iranians seeking to destroy vessels from land”.
Uranium raid
The US has a dedicated team of special forces ready to retrieve lost nuclear material. The National Mission Force, made up Delta Force and Navy Seals operators, has a designated “counter weapons of mass destruction mission” to recover loose uranium or “scattered nuclear related material that needs to get out of somewhere”, said Mr Hackett.
Usually this is a group of six to 12 soldiers with backpacks to carry the material out. But given the size of Iran’s stockpile, it would require a force of at least 1,000 personnel, Mr Hackett says.
It would also need engineers, earth-moving diggers and specialised equipment because the highly enriched uranium is dangerous to move. It would require a protection force that the marines, or more likely the 82nd Airborne, could provide.
