After a week of hostilities between the US and Iran raised fears of an all-out conflict that could draw in the two countries' global allies, President Donald Trump says Iran appears to be "standing down".

Key points: Iran lacks critical security guarantors if the US was to launch an offensive

Iran lacks critical security guarantors if the US was to launch an offensive Tehran largely has the support of Shia-led states including Iraq, Syria and Lebanon

Tehran largely has the support of Shia-led states including Iraq, Syria and Lebanon Experts say the ideal situation for Tehran would be an indirect Cold War through proxies

The latest escalation came when top Iranian military commander Qassem Soleimani was killed in a US drone strike near Baghdad airport last week, before Tehran hit back by firing missiles at Iraqi bases housing US troops.

Following the missile strikes, Foreign Minister Javad Zarif tweeted that Iran "took and concluded proportionate measures in self-defence" and did not "seek escalation or war".

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But while Iran-US tensions may be slightly easing for now, the underlying causes remain and the repercussions of the past week's events are still playing out.

We look at what a further conflict between the US and Iran could look like, which of their respective allies may or may not get involved, and why escalation into a World War III scenario is unlikely.

Who are Iran's allies?

Ordinary Iraqis are concerned about the repercussions of a US-Iranian war. ( Reuters: Essam al-Sudani )

The supreme leader, Sayyid Ali Hosseini Khamenei, is not just the leader of Iran but, as a Shia Marja or Ayatollah, he is also a religious figurehead for millions of Shia Muslims across the globe.

Regionally, Iran have provided political and military support to Shia governments and militia groups, who have in turn pledged their support to Iran.

Iraq, whose government has been dominated by Shia Muslims since the fall of Saddam Hussein, has held close diplomatic and military ties with both Iran and the US but recent attacks on Iraqi soil — viewed as a "violation of Iraq's sovereignty" — have damaged US-Iraqi relations.

Following Friday's attack near Baghdad airport that killed General Soleimani, the Iraqi Parliament passed a resolution calling for US forces to withdraw from Iraq.

Shia militia groups loyal to Iran, including Kataib Hezbollah, whose leader Abu Mahdi al-Muhandis was killed alongside General Soleimani, make up a significant part of Iraq's military.

Hezbollah has many supporters in Iran. ( Reuters via West Asia News Agency: Mehdi Bolourian )

Last week, elements of these groups spearheaded an attack on the US embassy and they have further threatened revenge against the US including ongoing attacks if they do not withdraw from Iraq.

Lebanon's Hezbollah, a political and military group which holds territory in Lebanon and positions in government, issued a statement condemning the killing of General Soleimani and called for a blockade against US goods and the closure of the US embassy in Baghdad.

Hezbollah has fought alongside Iran in Syria to support their mutual ally, the Assad Government.

Iran also back Houthi rebels, who control western Yemen, and Palestine's Hamas.

Iran may not be worth Russian and Chinese intervention

While Vladimir Putin (centre) has supported Syria's Bashar al-Assad (left), it remains unclear if Moscow will support Tehran. ( AP via Sputnik: Alexei Nikolsky, pool )

While Iran's Shia allies give them a wide reach regionally and a platform from which they could launch revenge attacks on US assets and pressure a withdrawal from Iraq through small-scale attacks, they lack strong alliances elsewhere.

Iran and Russia have been military allies in conflicts in Syria, Iraq and Afghanistan and also share an economic partnership.

China has also voiced determination to "develop a comprehensive strategic partnership with Iran" as tensions rise with the US.

While the US commands a large sphere of influence, the Middle East presents an uncertain landscape for Washington. ( ABC News: Jarrod Fankhauser )

Bryce Wakefield, executive director of the Australian Institute of International Affairs, told the ABC that in the case of an all-out conflict, Russia and China would keep a low profile.

"They may support Iran in the UN by voting against a Security Council resolution, sanctions or permission to use force on the part of the US," he said.

"But they're not going to want to overtly state that they support Iran."

Director of the Iran project for international non-profit Crisis Group, Ali Vaez, agreed, saying it was "highly unlikely China and Russia would be willing to pay a price in confronting the US to defend Iran".

"One of the reasons that Iran relies on this network of proxies and partners that it has developed over the years is because it is strategically alone. It doesn't have a security guarantor," he told the ABC.

And who's going to help out the US?

The US has various air, sea and land military assets in countries and waters surrounding Iran. ( US Navy: Mohamed Labanieh )

Militarily, the US far exceeds Iran in weapons capabilities and reach.

The US also have powerful political and military allies across the globe. But would these allies be willing to back them in an all-out conflict with Iran?

Regionally, the US is likely to gain support from key allies that would come under threat if tensions escalate.

On Wednesday, Israel's Prime Minister Benjamin Netanyahu praised Mr Trump for the killing of General Soleimani and said Israel would stand by the US.

Although small in landmass, Israel's military power is significant not only regionally but even on a global scale and their arsenal is believed to include nuclear weapons.

While Saudi Arabia has proved a strong ally of the US, particularly in the face of Iranian aggression, its reaction to the current build-up has been noncommittal to US interests.

Saudi Arabia's Vice Minister of Defence said on Wednesday the kingdom and its leadership "stand with brotherly Iraq" and will do everything in its power to spare it the danger of war.

Where does Europe stand?

EU leaders have called on Washington and Tehran to de-escalate tensions. ( AP: Virginia Mayo )

European allies have also called for de-escalation. The European Union and the French Foreign Minister both urged the continuation of dialogue to calm tensions.

Prime Minister Boris Johnson of Britain, speaking in the British Parliament on Wednesday, echoed the calls for calm but said that the Iranian general killed by the United States last week had "blood on his hands".

Dr Wakefield said while a full-scale conflict was extremely unlikely, if it did occur, the US would "put pressure on allies to commit troops, including Australia".

While some forces in Europe, most notably the UK, would likely support the US, other big players such as France and Germany would be unlikely to get involved militarily, he said.

But Dr Wakefield said such a scenario would not be in the interests of either country.

"An all-out war would mean the end of the Iranian regime, but it would also be extremely costly for the US in terms of lives lost and financial and other resources spent," he said.

The chance of WWIII? Not likely

The prospect of another US-led Middle Eastern war has triggered protests around the globe. ( AP: Aaron Favila )

While Iran-US tensions appear to be cooling compared to events of the past 48 hours, the overall risk of escalation has not diminished.

"The underlying causes are still there," Mr Vaez said.

"The reason we are where we are is the US maximum-pressure strategy has continued to squeeze the Iranian economy and has pushed the Iranians to take retaliatory measures of their own."

These measures include a staggered process of reducing their compliance with a 2015 nuclear deal as well as a string of attacks in the region on oil tankers, facilities of US allies and the most recent strikes on US bases in Iraq.

"That has brought us three times to the brink of a military confrontation between Iran and the US in the past six months," Mr Vaez said.

"As long as the maximum-pressure policy is still being implemented — and the Trump Administration is in fact signalling that it's going to double down on it — I think the source of tension is going to remain there and it's only a matter of time before the two countries are back at each other's throats again."

Millions reportedly came out to mourn Qassem Soleimani in Tehran. ( AP via Office of the Iranian Supreme Leader )

But the chances of escalation into a full conflict are thought to be minimal, with both sides repeatedly saying they do not want a war.

"I think the preferred option for the Iranians given the massive asymmetry in military capabilities between Iran and the US has always been an indirect Cold War that is basically carried out in proxy warfare," Mr Vaez said.

In the short term, Mr Vaez said, further attempts by Iranian allies to expel the US from Iraq, including harassment, rockets and roadside bombs may continue, while regionally shipping and infrastructure may also continue to be targeted.

Ayatollah Ali Khamenei (left) has vowed revenge for the killing of Qassem Soleimani (right). ( AP via Office of the Iranian Supreme Leader )

Dr Wakefield said the killing of Soleimani would have serious repercussions for the US.

"Trump's decision to exterminate what he says is a terrorist enemy of the United States has led to a situation where the United States will have less power and influence in the Middle East," he told the ABC.

The US is now left with the decision to either withdraw from Iraq or refuse the Iraqi Parliament's resolution.

If the US does withdraw its troops, Iran will reach its foreign policy goal of reducing US influence in the region.

"If the US don't withdraw it will play into an Iranian narrative that the United States is an imperial power in the Middle East," Dr Wakefield said.

"The killing of Soleimani was a decision taken without much strategic forethought at all. It seems to suggest that the United States has no respect for sovereignty in the region and this is a very powerful narrative that Iran will exploit."

It is also a powerful narrative that China and Russia are likely to exploit, and Mr Trump's threats to place further sanctions on Iran and destroy 52 of the country's cultural sites could reinforce such a narrative.