BAGHDAD — As the United States and its allies look to fight the Islamic State in Iraq and Syria, longtime adversaries with a common fear of the radical movement are scrambling to see if they can cooperate to defeat the rising threat.

The jihadist group known as ISIS has so far thrived in part because its enemies are also enemies of one another, a reality that has complicated efforts to muster a strong response to its rampage. That factor has been a crucial consideration in war planning in capitals as diverse as Tehran and Washington, London and Damascus. But the potential threat has also forced a re-examination of centuries old tensions between Sunnis and Shiites, Kurds and Turks.

“Everyone sees ISIS as a short-term nemesis,” said Vali Nasr, a former senior adviser at the State Department who is dean of the School of Advanced International Studies of Johns Hopkins University, adding that ISIS had thrust the region’s traditional set of rivalries into a “momentary pause.”

When the United States military was preparing to leave Iraq in 2011, its primary enemies were, for example, three Shiite militias, managed by Iran’s spymaster, Qassim Suleimani, and armed with bombs traced to factories in Iran. But recently, as United States warplanes bombed ISIS fighters closing in on an Iraqi town, Amerli, Mr. Suleimani directed three militias fighting the same enemy on the ground.

Iran and the United States insist there was no coordination, but the convergence of interests was a powerful symbol of just how much ISIS has, at least for now, reordered the region.

ISIS and its bloody march — mass killings, videotaped beheadings, ethnic cleansing — is forcing nearly every nation with a stake to reconsider relationships often shaped by competing agendas. 

Analysts say that following on the upheaval of the Arab Spring, the rise of ISIS has led to perhaps the most turbulent moment for the Middle East since the split centuries ago between Sunnis and Shiites.

“I don’t think there’s been anything like this since the seventh century,” said Daniel C. Kurtzer, a former American ambassador to Egypt and Israel who is now a professor at Princeton.

If there is one upside to the tumult, it at least offers the slim prospect of bringing greater stability to the fractured violent region by finding common ground among competing geopolitical, religious and ethnic differences.

But that may just as likely prove wishful thinking, experts said. Mr. Nasr suggested that Iran and the United States, for example, “have tactical convergence,” in Iraq, but show little chance of a more durable alliance.

“Iran is not going to get in the way of the U.S. going after ISIS,” Mr. Nasr said. “The U.S. is not going to get in the way of Iran going after ISIS.”

The Sunni power Saudi Arabia and Shiite Iran, whose long rivalry has shaped the sectarian divide of the Middle East and played out in proxy wars in Syria and Iraq, also find themselves both opposed to ISIS. This has raised hopes in the West of an opening in the fraught relationship between the two countries that could help not just defeat ISIS in Iraq, but perhaps help end sectarian skirmishes around the region, and resolve the three-year-old civil war in Syria.

But again Mr. Nasr said he saw only a reed of hope because, despite opposing ISIS, neither has given any indication that it is ready to give up a guiding principle of the two countries’ Middle East policies: that each opposes the other.

“Right now this is more hope in the West than a reality on the ground in the Middle East,” said Mr. Nasr of a potential thaw in relations between Saudi Arabia and Iran.

The complex landscape of shifting alliances is particularly acute in Syria, where ISIS rose in the vacuum of the civil war before sweeping across Iraq. As President Obama weighs widening a military campaign against ISIS by taking on the group inside Syria, he faces an even more complex situation than in Iraq, where there are obvious allies to do the fighting on the ground, including the Iraqi security forces, the Kurdish pesh merga, and the Iranian-backed Shiite militias.

In Syria, the United States has called for the ouster of President Bashar al-Assad, while Iran has supported him. Russia, which has increasingly angered the West with its military involvement in Ukraine, is also another important ally of Mr. Assad. So Mr. Obama has to calculate how to fight ISIS without appearing to aid Mr. Assad and the agenda of Iran and Russia. If he helped the Syrian president, even indirectly, he would violate his own stated objective and anger Turkey, an important American partner in the region that has long opposed Mr. Assad.

“A year ago we were discussing if we were going to bomb this guy or not,” said Michael Stephens, an analyst in Qatar for the London-based Royal United Services Institute. “Now we’re talking about helping him out.”

Turkey has its own set of dilemmas. Early in the Syrian uprising Turkey bet that Mr. Assad would go quickly, and supported rebel groups with weapons and offered them the ability to go back and forth across its long border with Syria. As the uprising evolved into a brutal civil war, Turkey was sharply criticized for allowing ISIS to grow.

Now ISIS has become a threat to Turkey, and holds nearly 50 Turkish hostages captured when the militants overran the Turkish Consulate in Mosul, Iraq’s second largest city, in June.

So while Turkey is eager to defeat the scourge of ISIS, it has been restrained out of worry for the lives of the hostages. Publicly, Turkish leaders have opposed the American airstrikes in Iraq.

“Privately, it’s a very different story,” said Sinan Ulgen, a former Turkish diplomat and chairman of the Center for Economic and Foreign Policy Studies, a research organization in Istanbul. “Turkey wants a more effective operation against ISIS.”

Turkey also has another important interest. The Kurdistan Workers’ Party, or P.K.K., has long fought an insurgency against Turkey, but after militants threatened Iraq’s Kurdish region — which prompted the first American airstrikes — its fighters swooped in to the fight in defense of the Kurds. The P.K.K. has long been on the United States terrorist list, but now there are calls for the group to be removed, which would enhance the group’s legitimacy, and anger the Turks.

Broadly, the rise of ISIS has sped up a process of reshaping the Middle East that began with the Arab Spring uprisings over three years ago. Across the region, the old order disintegrated.

“You basically had an open field for these regional rivals to fight over, and the fighting is not over,” Mr. Nasr said. “All of this put together is a consequence of the Arab Spring.”

Separate from ISIS, the Arab Spring brought to power the political Islamists in many countries, sharpening a rivalry between Saudi Arabia, the United Arab Emirates and Egypt, all opposed to the Muslim Brotherhood, and Qatar and Turkey, supporters of the Brotherhood.

This rivalry has played out in Libya, where the U.A.E., with Egyptian support, has bombed Islamists. Meanwhile, Israel is “quite gleeful,” Mr. Kurtzer said, to have the tacit support of Egypt, Saudi Arabia and the U.A.E. in its effort to crush Hamas, the militant group that is an offshoot of the Muslim Brotherhood and is the authority in the Gaza Strip.

The new reality in the region is personified by the position Hakim al-Zamili, an Iraqi politician and militia leader, finds himself in these days. Mr. Zamili, a top official in the movement led by the Shiite cleric Moktada al-Sadr, was once in an American detention facility, accused of leading a Shiite death squad during Iraq’s sectarian civil war in 2006 and 2007.

He was also on the ground in Amerli, along with Mr. Suleimani, the top Iranian operative.

“We have had no problems with the U.S. since they withdrew from Iraq,” he said. “I fought against them, as they were invaders. But today they are not. We are now allied to fight ISIS together.”