The killing of top Iranian general Qassem Soleimani, ordered by United States President Donald Trump, has renewed fears of war between the United States and Iran.

Key points: Iran's military is assigned the "ideological mission of jihad", extending "God's law" overseas

Iran's military is assigned the "ideological mission of jihad", extending "God's law" overseas Its missile arsenal and regional network of proxies are strengths for Tehran

Its missile arsenal and regional network of proxies are strengths for Tehran Many warn a US war with Iran would create another decades-long quagmire

Following Friday's attack, Iran's Supreme Leader Ayatollah Ali Khamenei has vowed "severe revenge" over General Soleimani's death, while the US has announced it will deploy an additional 3,500 troops to the Middle East.

Mr Trump said the US would "hit very fast and very hard" some 52 Iranian sites — including those important to "the Iranian culture" — if Tehran were to retaliate.

"God the almighty has promised to get his revenge, and God is the main avenger. Certainly, actions will be taken," General Soleimani's replacement, general Esmail Ghaani, has declared.

Lightning-speed escalation between the two ideological juggernauts has left analysts scrambling to decipher the situation.

"In short, the United States has taken a highly escalatory step in assassinating one of the most important and powerful men in the Middle East," longtime Iran expert Ilan Goldberg wrote over the weekend.

"The most important question now is how will Iran respond."

But would the US and Iran really enter direct military conflict with one another?

Iran's Supreme Leader Ayatollah Ali Khamenei said his country would take "severe revenge". ( Supplied: leader.ir )

What consequences might an Iranian-American conflict have for the Middle East and the world?

Here's a breakdown of what we know about the two militaries and what we might expect.

How do Iran and the United States' militaries compare?

At face value, the two nations' military capabilities and economies seem worlds apart.

The Global Firepower index ranks the US as the most powerful military in the world with Russia second and China third. Iran is ranked number 14 below Turkey and even Egypt.

The US Armed Forces is significantly more powerful than Iran's military. ( Reuters: US Air Force )

In 2018, Iran spent $US13.2 billion ($18.9 billion) on its military while the United States spent a whopping $US648.8 billion ($933.6 billion), according to the Stockholm International Peace Research Institute (SIPRI).

By comparison, Australia spent $US26.7 billion ($38.4 billion).

The US Government estimates there are approximately 600,000 active members of the Iranian armed forces and some 500,000 to 1 million potential reservists.

By comparison, the US Armed Forces have more than 1.3 million active service personnel and more than 800,000 reservists.

Tehran and Washington have spent months trading attacks and threats across the wider Middle East. ( AP: Hasan Shirvani via Mizan News Agency )

However, military conscription is mandatory in Iran for all males over the age of 18, hence the difficulty in knowing exactly who might qualify as an active reservist.

Iran's military consists of two major institutions: the regular forces — known as Artesh — and the Islamic Revolutionary Guard Corps.

The Iranian Constitution of 1979 declares that the country's defence forces are an "ideological army" responsible not only for national security but to be proponents of "God's law" throughout the world.

As such, the Revolutionary Guard is the single most important and influential institution in Iran, with profound influence over civilian life and a key role in spreading Iran's influence overseas.

However, the US has a vastly greater overall population and the world's most advanced military apparatus, with a history of using nuclear weapons.

It also remains allied with the most powerful and prosperous nations not only regionally, but around the world.

What are Iran's strengths and weaknesses?

US Defence Intelligence Agency director Vincent R Stewart recently identified Iran as one of the five top military threats facing the nation, and key to Iran's military clout is its arsenal of missiles.

According to the Centre for Strategic and International Studies in Washington, Iran possesses the largest and most diverse missile arsenal in the Middle East, with some of its ballistic and cruise missiles able to strike Israel over 2,000 kilometres away as well as parts of south-east Europe.

Economic sanctions have frustrated Iran's nuclear ambitions. ( AP: Ebrahim Noroozi )

But while Tehran has repeatedly launched missiles into Syria and Iraq in recent years and been accused of attacking Western-allied oil tankers, it has never launched a missile into either Israel or Europe.

This is despite its threats to "wipe Israel off the face of the Earth".

Many analysts maintain this is due to the reality that Iran would not be able to endure a conflict with the US, due to an ailing economy and the fact that most of its allies are non-state actors.

Iran engages in proxy warfare across the Middle East by supporting certain groups and Governments, but avoids direct conflict. ( ABC: Steven Viney )

Nevertheless, Tehran has demonstrated an ability to bolster its military clout through an extensive network of proxies across the region — mostly Shiite militias that are hostile to the US, Israel and Saudi Arabia.

It is believed to have long supplied weapons and support to Hezbollah in Lebanon, in addition to Houthi rebels in Yemen as well as Bashar al-Assad's regime in Syria.

Iran's missile arsenal is the largest in the Middle East. ( AP via ISNA: Amir Kholousi, file )

General Soleimani was the leader of the Quds Force, the unit within the Revolutionary Guard responsible for overseas operations, including the establishment and support of proxies.

And his replacement General Ghaani has loudly promised revenge against the US.

Meanwhile, despite being hampered by decades of international sanctions, Iran remains committed to modernising its military.

It has signalled its intention to continue its uranium enrichment program after former president Barack Obama's signature 2015 nuclear deal fell apart under Mr Trump.

Iran is honing its use of drones for military purposes. ( AP: Tasnim News Agency )

Iran has also made rapid progress in developing its capabilities with unmanned aerial vehicles (UAVs) — for example, a warhead-carrying Raad 85 UAV sometimes referred to as a "suicide drone" — which can be directed at a target by a human operator.

Nuclear weapons aside, CSIS expert Anthony H Cordesman has argued that ballistic missiles and UAVs have the potential to be "weapons of mass effectiveness" in the hands of Iran.

What about the United States' strengths and weaknesses?

While Iran boasts a large missile arsenal, its existence is designed to compensate for one major arsenal disadvantage — the country's relatively weak air force and lack of regional military bases and key allies.

The US has a vastly superior airforce to Iran. ( Getty: Justin Sullivan )

Despite largely pulling out of Iraq in recent years, the US maintains some 800 military bases around the world.

It has important regional allies like Saudi Arabia, the UAE, Turkey, Egypt and Israel, all of which have the latest Western-made aircraft, giving them a significant technological edge over the Islamic Republic.

Israel is believed to be the only nuclear-armed nation in the region, possessing an estimated 80-90 warheads. The United States has thousands.

US allies control trade routes like the Suez Canal — in the case of Egypt — and resources like oil — in the case of Saudi Arabia.

Qassem Soleimani was a popular figure among Shiites across the Middle East. ( AP: Ebrahim Noroozi )

Iran's regional allies, meanwhile, are generally confined to Lebanon, Syria, and Kuwait.

And due to years of sanctions, Tehran has been stymied in its efforts to import new aircraft from suppliers like France or Russia.

"Iran is one of the most strategically lonely countries in the world," Iran specialist Karim Sadjadpour told the WSJ recently.

"It considers dozens of countries around the world its adversary, and its only reliable friend has been the Assad regime in Syria."

So even though Iran has a large missile arsenal and an apparently improving air defence capacity with various proxies, Iran remains highly vulnerable to the vastly superior firepower of the US, whether directly or indirectly through allies and proxies.

While Iran's nuclear program has caused anxiety for the US, it has not yet successfully developed nuclear weapons capabilities, and analysts maintain it is still far off.

Iran's nuclear industry was dealt a significant blow in 2010 when a computer virus called Stuxnet — developed by the US and Israel — infected more than a dozen Iranian nuclear facilities.

The attack shut down 1,000 nuclear centrifuges across the Islamic Republic and accidentally revealed that Washington had been running a sophisticated cyber operation against Iran.

How might a conflict play out?

Iran would clearly be at a considerable disadvantage in the event of a direct conflict with the United States.

Neighbouring Iraq had the fourth most powerful military in the world when it invaded Kuwait and launched missiles against Saudi Arabia and Israel in 1990.

Yet the Gulf War and subsequent US invasion proved it was no match for Washington's military might.

It is thus difficult to predict how Iran might seek to react to the killing of General Soleimani.

The risk of retaliation may make it hard for Iran to respond to Mr Trump's military action. ( AP: Patrick Semansky )

Ian Parmeter, a former Australian ambassador to Lebanon and a researcher with the ANU's Centre for Arab and Islamic Studies, said Iran's leaders would be under "enormous pressure" to respond militarily.

"Iran can't not respond, it has to respond. But it needs to be clever about how it does it, given Trump has already promised to retaliate," he told the ABC.

"The question is what they can do without bringing enormous American firepower back upon them."

In an article for Foreign Affairs, analyst Ilan Goldenberg wrote Iran was likely to be fairly cautious in its response and may not "rush to retaliate".

However, he said the retaliation was likely to take place in Iraq, given that was where the killing of Soleimani took place, and would involve pro-Iran militias, which he said were "among Iran's most responsive proxies".

He predicted both countries would ultimately seek to avoid an all-out war but that "the risk of miscalculation is incredibly high".

Mr Parmeter echoed that the response would likely involve Iran's regional proxies, however, added that it remained unclear whether this could provoke further retaliation from Mr Trump.

Either way, there was no clear way forward for Tehran, which he said had found itself in "a very difficult place".

"There's no easy way for them to retaliate, but if they don't retaliate, they really do look very weak — not just in the eyes of their own people, but in the region in terms of the states in the region they want to impress," Mr Parmeter said.