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A conflict with Iran would not be like the Iraq War. It would be worse.

An F/A-18E Super Hornet launches from the flight deck of the aircraft carrier USS Abraham Lincoln on May 10 in the Red Sea. (Mass Communication Spec. Seaman Michael Singley/U.S. Navy/Getty Images)

This story initially published May 14, 2019. It has been updated.

After the United States claimed responsibility for killing Qasem Soleimani, a top Iranian military figure, in a drone strike in Baghdad early Friday, Iran said Washington could expect “severe revenge.”

The attack, a dramatic escalation in tensions between Iran and the United States, sent waves of anxiety across the region. Americans in Iraq were urged to leave the country immediately. Tensions soared across the Middle East as the White House defended the strike, with President Trump saying he ordered the attack against Soleimani because the Iranian commander was planning imminent attacks against Americans. European allies urged de-escalation, voicing concerns that Soleimani’s killing could spark a new war.

Trump says fatal airstrike was meant ‘to stop a war,’ warns Iran not to retaliate

Tensions between Iran and the United States have increased since Trump withdrew from the 2015 nuclear agreement with Iran, and some of those tensions played out in Iraq in recent weeks between Iraqi militias allied with Iran and U.S. forces stationed there.

On Friday, Trump said the United States took action “to stop a war.”

“We did not take action to start a war,” he said.

In recent months, the escalating dispute between a U.S. administration led by a tough-talking Republican president and an embattled but antagonistic Middle Eastern power has reminded many observers of the run-up to the U.S.-led invasion of Iraq in 2003. In the years since, that operation has been widely condemned as disastrous for all involved.

Even some of the characters in the apparent remake were the same: John Bolton, Trump’s former national security adviser, played a key role in President George W. Bush’s buildup to the Iraq invasion as undersecretary of state for arms control and international security.

Bolton’s actions at the time earned him a reputation as reckless. He was ousted from the Trump administration in September.

Despite the similarities, a conflict with Iran would not simply be a redux of the 2003 war with Iraq. It would be quite different in many ways — and it would almost certainly be substantially worse. Present-day Iran is a significantly different country compared to Iraq in 2003. The way it would fight a war is very different, too.

If nothing else, Iran is a bigger country than Iraq was before the 2003 invasion. At the time, Iraq’s population was about 25 million. Iran’s population is estimated to be more than 82 million. Iran spans 591,000 square miles of land, compared with Iraq’s 168,000 square miles.

One estimate from 2005 suggested the Iraqi army had fewer than 450,000 personnel when the U.S. invasion began. Recent estimates suggest that Iran has 523,000 active military personnel, as well as 250,000 reserve personnel.

Just as important, however, is Iran’s location. Unlike Iraq, Iran is a maritime power bordered by the Caspian sea to the north and the Persian Gulf and the Gulf of Oman to the south. It shares land borders with several troubled U.S. allies, including Afghanistan, Pakistan, Turkey and Iraq.

Its location in the center of Eurasia is particularly important for trade. About a third of the world’s oil tanker traffic passes through the Strait of Hormuz, which is bordered by Iran and Oman. At its narrowest point, this shipping route is just under two miles wide. Blocking it could lead shipments of daily global oil exports to drop by an estimated 30 percent.

[Trump ordered attack on Iran for downing drone, then called it off, citing casualties]

In terms of conventional military strength, Iran is far weaker than the United States. But the country has long pursued asymmetric strategies that could allow it to inflict serious damage on U.S. interests in the region.

Iran’s Revolutionary Guard Corps, a force loyal to Iran’s supreme leader, Ayatollah Ali Khamenei, and separate from the regular military, has an external special operations arm known as the Quds Force that has helped build proxy forces in places such as Iraq, Lebanon and Syria. It funds militias such as Hezbollah, which is powerful in its own right. Soleimani, who was killed Friday, was the commander of the Quds Force and was closely aligned to Iraqi militias.

Iran has used these type of groups to target Americans before. In 2018, a revised Pentagon estimate found Iranian proxy forces had killed at least 608 U.S. troops in Iraq between 2003 and 2011. Iranian proxies could cause havoc in Iraq and Afghanistan again.

Iran’s navy has a real advantage against the United States, too. It doesn’t need big ships or firepower to block off the Strait of Hormuz, for example, but could use mines or submarines to force a halt in trade.

U.S. war games have suggested that speedboat suicide attacks and missiles could be surprisingly effective against the American military. A 2017 report from the Office of Naval Intelligence found that the navy of the Revolutionary Guard, which is distinct from Iran’s regular navy and focuses on smaller and faster but still heavily armed vessels, had been given more responsibilities to protect the Persian Gulf.

Under President Trump, United States-Iranian relations have taken a decisive turn for the worse. Here's a brief history of the tumultuous relationship. (Video: Joyce Lee/The Washington Post)

Then there’s Iran’s ballistic missile program, which the Missile Threat Project at the Center for Strategic and International Studies describes as “the largest and most diverse missile arsenal in the Middle East.” The threat from Iranian missile technology extends beyond the country’s borders: Hezbollah is thought to have an arsenal of 130,000 rockets.

If the United States were to engage in a military conflict with Iran, it would probably require significant manpower that would otherwise be used to check bigger powers such as China or Russia. The New York Times reported last year that Trump’s acting defense secretary at the time, Patrick Shanahan, drew up plans to deploy 120,000 U.S. troops to the region if Iran attacked American forces or restarted its nuclear program; this was based on a scenario that did not involve an invasion, which would require more troops.

The invasion of Iraq involved 150,000 U.S. troops, along with tens of thousands from allied nations. The financial cost of the Iraq War was pegged at over $2 trillion in 2013, with about 400,000 people estimated to have been killed between 2003 and 2011.

American military planners know all this. However, the U.S. government cannot say that there are no good options to militarily engage Iran, because doing so would take the threat of military action off the table and diminish the pressure that it hopes to maintain on Tehran. It’s a risky strategy that has even some of America’s closest allies worried.

Read more:

The United States meant to isolate Iran. It looks increasingly isolated itself.

U.S. conflict with Iran: What you need to read

Here’s what you need to know to understand what this moment means in U.S.-Iran relations.

What happened: President Trump ordered a drone strike near the Baghdad airport, killing Maj. Gen. Qasem Soleimani, Iran’s most powerful military commander and leader of its special-operations forces abroad.

Who was Soleimani: As the leader of the Revolutionary Guard Corps’ elite Quds Force, Soleimani was key in supporting and coordinating with Iran’s allies across the region, especially in Iraq. Soleimani’s influence was imprinted on various Shiite militias that fought U.S. troops.

How we got here: Tensions had been escalating between Iran and the United States since Trump pulled out of an Obama-era nuclear deal, and they spiked shortly before the airstrike. The strikes that killed Soleimani were carried out after the death of a U.S. contractor in a rocket attack against a military base in Kirkuk, Iraq, that the United States blamed on Kataib Hezbollah, an Iran-backed militia.

What happens next: Iran responded to Soleimani’s death by launching missile strikes at two bases hosting U.S. forces in Iraq. No casualties were reported. In an address to the nation, Trump announced that new sanctions will be imposed on Tehran.

Ask a question: What do you want to know about the strike and its aftermath? Submit a question or read previous Q&As with Post reporters.