Editor's Note: This article originally appeared on Business Insider.

President Donald Trump ordered the killing of Iran's top military commander, Maj. Gen. Qasem Soleimani, in an airstrike last week, sparking a major escalation in tensions with Iran.

While experts widely debated Iran's anticipated response, the country took action on Tuesday night by firing more than a dozen ballistic missiles at two bases in Iraq hosting U.S. and Iraqi military personnel.

The missile strikes did not result in casualties, a move that many observers suspect was intentional in order to retaliate against the U.S. without escalation into a full-blown war.

Trump on Wednesday touted the fact that the missile attacks resulted in zero American or Iraqi casualties and left minimal damage to the Erbil and al-Asad facilities as something “the American people should be extremely grateful and happy” for.

He added that Iran “appears to be standing down,” and vowed that the days of tolerating Iran's malign behavior “are over.”

But some experts have raised concerns that tensions between the U.S. and Iran may actually embolden a common enemy — ISIS — which may seek a resurgence amid the chaos created in Iraq.

Soleimani played a key role in the fight against the ISIS in Iraq

Iranian Maj. Gen. Qassem Soleimani at a meeting with Supreme Leader Ayatollah Ali Khamenei and Revolutionary Guard commanders in Tehran, Iran, in 2016. (Office of the Iranian Supreme Leader via Associated Press;p90)

While the United States and Iran have had no formal diplomatic relations since 1980, the two found unlikely common ground in the battle against ISIS.

The U.S.-led military coalition helped train Kurdish fighters on the ground, as well as conduct airstrikes against Islamic State positions in both Iraq and Syria.

Iran, too, was deeply involved in the fight against Islamic State. In Syria, the country supported the Assad government since the start of the Syrian Civil War in 2011, and in Iraq, the Iranian government was among the first to pledge assistance in battling the extremist group, deploying its own troops on the ground.

Iran's participation in the fight against ISIS largely stemmed from the country's desire to have more influence in the Middle East. However, it is also linked to the centuries long ideological fight between two sects of Islam — Shia and Sunni — which have been at ideological odds for centuries.

ISIS practices an extreme form of Sunni Islam called Salafism, and does not consider Shia Islam — which is the dominant strand of Islam in Iran and large portions of Iraq — to be true Islam.

Sunni Muslims, who consider themselves to be a more orthodox branch of Islam, represent a majority of the world's Muslims, making up about 90% of the populations of Egypt, Jordan and Saudi Arabia.

Shia Muslims represent a smaller population of the world's Muslims, though they are the majority in some Islamic countries, including Iran.

This sectarian conflict has historically been linked to many crises in the Middle East over the years, particularly because Iran has adopted a policy of supporting Shia militia and governments through a vast network of proxies, putting it at odds with Sunni powers like Saudi Arabia.

Leaked documents published by The Intercept in November 2019 revealed a vast network of Iranian influence in Iraq, largely orchestrated by Soleimani.

These are the forces across the region that the Quds Force is known to support via things like advice, training, and armaments to spread Iranian influence and threaten adversaries like Israel and Saudi Arabia. (Business Insider/Skye Gould)

Iran photo

The Sunni-Shia divide flared in the last decade as ISIS gained power in Syria and Iraq.

Soleimani rose to prominence during this time by advising Kurdish and Shia forces in fighting Islamic State together. In 2014, Soleimani was credited with helping the Iraqi city of Amirli withstand an ISIS assault.

And although Iraqi Prime Minister Haider al Abadi declared victory over the Islamic State in December 2017, a recent report indicates that ISIS may still have between 14,000 and 18,000 members in Iraq and Syria, including up to 3,000 foreigners.

The group has signaled that followers of its ideology continue to exist, particularly during periods of unrest in its former strongholds. After Trump hastily pulled U.S. troops out of Syria in October, the group took advantage of the chaos and launched an attack at an Iraqi oil field, prompting concerns of resurgence.

The battle between Iran and the US playing out on Iraqi soil could give ISIS 'greater freedom of action'

Experts have warned that the conflict playing out between the United States and Iran will only fuel further instability in Iraq, which is slowly recovering from the devastation brought on by the emergence of ISIS within its borders.

Lydia Khalil, a research fellow at the Lowy Institute in Sydney, told Business Insider that the fact that the conflict between the U.S. and Iran is playing out on Iraqi soil is a “complicating factor.”

“Iraq has been a theatre for U.S.-Iranian competition, influence and power projection for some time,” she said. “Iraq will remain and increase as a battlefield between US and Iran after this event which will complicate Iraq's stabilization and governance.”

Rodger Shanahan, also a research fellow at the Lowy Institute, said that considering Soleimani's “significant” role in battling Islamic State in Iraq, his death has reverberated through the region.

For followers of the Islamic State in Iraq, Soleimani's death, along with the ensuing chaos on the ground, give them a window of opportunity to strike.

“They are looking to survive and carve out areas of influence within Iraq if possible,” Shanahan said of Islamic State supporters following Soleimani's killing.

Shanahan said that a total Islamic State resurgence would be surprising, and that Soleimani's death is “unlikely to embolden whatever supporters Islamic State has elsewhere,” which include active franchises in West Africa and Afghanistan.

But in Iraq — where the roots of the Islamic State were established in 2004 — the group has a chance at revival.

“Iraq is their heartland,” Shanahan said. “Any diminution in coalition or Iraqi military actions will give them greater freedom of action.”

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