Hours after the Trump administration announced the killing of a top Iranian official, "World War III" became a top-trending search term.

Traffic to the US Selective Service, the independent government agency that maintains a database of Americans eligible for a potential draft, spiked so high that the website crashed.

"Due to the spread of misinformation, our website is experiencing high traffic volumes at this time," the agency said on Twitter, adding, "We appreciate your patience."

On the surface, at least, these concerns would seem justified. The US and Iran have been trading threats for more than a week, and earlier today, US President Donald Trump warned of a "disproportionate" military response to Iran if it carries out its threat of a retaliatory strike on the US.

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Taking to Twitter, he said his social media posts "will serve as notification to the United States Congress that should Iran strike any US person or target, the United States will quickly & fully strike back, & perhaps in a disproportionate manner".

It came after Iraq's foreign ministry denounced the US drone attack as a "blatant" violation of sovereignty and a breach of the agreement between Iraq and the US-led coalition.

Iran has now announced it will no longer abide by the limits of the 2015 nuclear deal, and Iraq's parliament called for the expulsion of all American troops from Iraqi soil.

These developments could bring Iran closer to building an atomic bomb and enable the Islamic State group to stage a comeback in Iraq, making the Middle East a far more dangerous and unstable place.

HOW DID WE REACH THIS POINT?

The US and Iran have been in a flare-up for more than a week with a series of back-end-forth strikes.

On December 27, an American contractor was killed after more than 30 rockets were launched at an Iraqi military base.

Two days later, the US responded with air strikes on sites in Syria and Iraq, killing 24 members of an Iranian-backed militia.

On December 31, pro-Iranian militia members laid siege to the US Embassy in Baghdad, trapping American diplomats for more than 24 hours.

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But January 3 marked the biggest escalation of all when the Trump administration launched a drone strike killing Qassem Soleimani.

Soleimani was commander of Iran's Quds Force, the most important branch of the Iranian military forces; a man who's led Iran's campaign across the Middle East, including a counter-IS campaign.

Mourners step over US flags with pictures of President Donald Trump, under a picture of assassinated general Qasem Soleimani. Photo / AP
Mourners step over US flags with pictures of President Donald Trump, under a picture of assassinated general Qasem Soleimani. Photo / AP

Tehran immediately warned of retaliation, stating: "Soleimani's martyrdom will make Iran more decisive to resist America's expansionism and to defend our Islamic values. With no doubt, Iran and other freedom-seeking countries in the region will take his revenge."

The following day, on January 4, two rockets hit Iraq's Al-Balad air base where US troops are stationed, and two mortars hit Baghdad's Green Zone that houses the US Embassy.

Today, a fresh pair of rockets were launched on the same area.

A third rocket simultaneously hit a family home outside the Green Zone, wounding four, medical sources told AFP.

WILL THIS LEAD TO WAR?

War threats have long been exchanged between the US and Iran, and increasingly so over the past two years.

The assassination of Soleimani, one of the most powerful men in the Middle East, marked a sharp escalatory step.

Thousands in Tehran took to the streets chanting "Death to deceitful America". Iranian politicians chanted "Death to America" during an emergency parliament session called after his assassination.

No one is in doubt that a war between the US and Iran would be disastrous for both sides.

"Americans would be wise to brace for war with Iran," Susan Rice, who served as national security adviser from 2013 to 2017, wrote in The New York Times. "Full-scale conflict is not a certainty, but the probability is higher than at any point in decades."

She said Iran's response would be "multifaceted" and "unpredictable", warning that if Mr Trump reacts with additional force – as would be expected – the confrontation is likely to "spiral into a wider military conflict".

But while the retaliation from Iran is expected to be significant, this is unlikely to mark a World War III-style scenario.

For one thing, no government allies of either country would be willing to join in such a massive conflict.

At the same time, Russia and China are highly unlikely to join the fight. "While the possibility of an unintended slide to war is impossible to rule out, fears of World War Three — a phrase that trended overnight on social media — are overblown," writes Max Fisher in the Times. "Russia and China might strenuously object to American attacks, but they are no more likely to join the fight than they were when the United States invaded Iraq or helped to topple Libya's government."

IRAN LIKELY TO RESPOND WITH CYBERATTACKS

Iran's retaliation for the assassination of Soleimani is likely to focus on cyberattacks, security experts have warned.

The country's state-backed hackers are already among the world's most aggressive and could inject malware that triggers major disruptions to the US public and private sector.

Potential targets include manufacturing facilities, oil and gas plants and transit systems.

A top US cybersecurity official is warning businesses and government agencies to be extra vigilant.

History has shown that Iran possesses strong cyber capabilities. In 2012 and 2013, in response to US sanctions, Iranian state-backed hackers carried out a series of disruptive denial-of-service attacks that knocked offline the websites of major US banks including Bank of America as well as the New York Stock Exchange and NASDAQ. Two years later, they wiped servers at the Sands Casino in Las Vegas, crippling hotel and gambling operations.

The destructive attacks on US targets ebbed when Tehran reached a nuclear deal with the Obama administration in 2015.

The killing of Soleimani – long after Trump scrapped the nuclear deal – completely alters the equation.

"Our concern is essentially that things are going to go back to the way they were before the agreement," said John Hultquist, director of intelligence analysis at the cybersecurity firm FireEye. "There are opportunities for them to cause real disruption and destruction."

Iran has been doing a lot of probing of critical US industrial systems in recent years – trying to gain access – but has limited its destructive attacks to targets in the Middle East, experts say.

It's not known whether Iranian cyberagents have planted destructive payloads in US infrastructure that could now be triggered.

"It's certainly possible," Hultquist said. "But we haven't actually seen it."

Iran has been increasing its cyber capabilities but is not in the same league as China or Russia – which have proved most adept at sabotaging critical infrastructure, witnessed in attacks on Ukraine's power grid and elections, experts agree.

CONFLICT HAS BEEN A LONG TIME COMING

While the assassination of Soleimani marked a massive escalation in US-Iran tensions, it didn't come from nowhere.

In May 2018, Trump withdrew from the Joint Comprehensive Plan of Action, or "Iran deal", by which Tehran agreed to scrap its uranium and nuclear-building facilities in exchange for the lifting of crippling economic sanctions.

Trump said it was a "horrible one-sided deal" that gave Iran too much power.

In June last year, Iran shot down a US military surveillance drone, destroying an expensive piece of equipment.

Body of top Iran General, Qassem Soleimani passed over thousands of mourners. Video / 9news

The US military responded, targeting Iran with a cyberattack that disrupted Iran's missile program, and Trump traded insults with Iranian officials over the events.

In September last year, Iranian Foreign Minister Javad Zarif threatened "all-out war" on the US.

It came a day after US Secretary of State Mike Pompeo accused Iran of carrying out an "act of war" with the air strikes.