BEIRUT, Lebanon — Days after seizing the Syrian desert city of Palmyra, Islamic State militants blew up the notorious Tadmur Prison there, long used by the Syrian government to detain and torture political prisoners.
The demolition was part of the extremist group’s strategy to position itself as the champion of Sunni Muslims who feel besieged by the Shiite-backed governments in Syria and Iraq.
The Islamic State, also known as ISIS or ISIL, has managed to advance in the face of American-led airstrikes by employing a mix of persuasion and violence. That has allowed it to present itself as the sole guardian of Sunni interests in a vast territory cutting across Iraq and Syria.
Ideologically unified, the Islamic State is emerging as a social and political movement in many Sunni areas, filling a void in the absence of solid national identity and security. At the same time, it responds brutally to any other Sunni group, militant or civilian, that poses a challenge to its supremacy.
That dual strategy, purporting to represent Sunni interests and attacking any group that vies to play the same role, has allowed it to grow in the face of withering airstrikes.
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The Islamic State aims to build a broad colonial empire across many countries.
While the Obama administration has focused on confronting the Islamic State militarily, experts say the group’s recent victories point to the need for a political component in the strategy to counter the group — even after nearly 4,000 airstrikes by the American-led coalition and what United States officials say are the deaths of 10,000 ISIS militants.
“They are hijacking legitimate demands,” Ibrahim Hamidi, a journalist and political analyst from the restive Syrian province of Idlib, said of the Islamic State. He has long argued that without more forceful international action against the Syrian president, Bashar al-Assad, and a political program to empower Sunnis, support for the extremists will grow.
“This is part of what makes them so dangerous,” he said. “Defeating them needs a comprehensive approach.”
Sunnis form an aggrieved majority in Syria, where repression of a mostly Sunni uprising by Mr. Assad’s government, backed by Shiite-led Iran, exploded into war. And they are an aggrieved minority in Iraq, where Shiites are more numerous, after years of sectarian violence between Sunni and Shiite militias.
But little has been done to give Sunnis a greater role in their own governance. Mr. Assad remains in power, backed by Iran and the militant group Hezbollah. And American officials are fighting an uphill battle to persuade Sunnis in Iraq to fight ISIS alongside the Shiite-led central government and Iranian-backed militias.
That, Mr. Hamidi and other analysts said, has left some Sunnis willing to tolerate the Islamic State in areas where they lack another defender, especially in conservative communities like the ones in western Iraq and eastern Syria, where the group is strongest. The analysts emphasized that most Sunnis do not support the Islamic State’s harsh interpretation of Islam, or its brutality, but that some were becoming more susceptible to its political talk about protecting oppressed Sunnis.
“Now, with the sectarian polarization of the region, under the skin of every single Sunni there is a tiny Daesh,” Mr. Hamidi, a Sunni, said, using the Arabic acronym for the Islamic State.
By attacking ISIS in Syria while doing nothing to stop Mr. Assad from bombing Sunni areas that have rebelled, he added, the United States-led campaign was driving some Syrians into the Islamic State camp. “The coalition is scratching the skin and making this Daesh come out.”
ISIS has more than enough in its coffers despite expectations that airstrikes and falling oil prices would hurt the group.
A slang word has even emerged, aid workers in Palestinian refugee camps in Lebanon say, for someone who supports ISIS just a little bit. Some people say, with a hint of sheepishness, “I’m a Dawoosh,” using an Arabic diminutive that suggests “a cute little Daesh.”
In Iraq, the Islamic State has largely engulfed a pre-existing Sunni insurgency. It has deeper roots there, having grown out of Al Qaeda in Iraq, that sprang from resistance to United States after the 2003 invasion.
In 2006, Iraq became embroiled in civil war, with Sunni and Shiite militias carrying out sectarian attacks on civilians. Now, some of those Shiite militias are among the most effective forces on the ground after the partial collapse of the Iraqi Army. But there have been reports of revenge killings by the militias, and some Sunnis trust them so little that they prefer the Islamic State.
The situation in Syria is similar in some ways, with Mr. Assad relying on support from Iran and Iranian-backed militias. But the Islamic State faces greater challenges winning popular support in Syria, where its Iraqi leadership is viewed as foreign and where there remains a significant collection of Sunni insurgent groups opposed to both the Islamic State and the government.
Washington is trying to recruit some of those rival groups to be trained and equipped as a front-line force against the Islamic State in Syria. But the program is small, with only 90 fighters in the first round of training, and recruiting is a challenge, since most Syrian insurgents place the highest priority on fighting the Assad government.
Also arrayed against the Islamic State in Syria are many hard-line Islamist groups, including the Qaeda-affiliated Nusra Front. They can match the Islamic State in espousing Sunni sectarian views, condemning minority sects, and reject its claim to represent Sunnis, calling them instead “khawarej,” a term from Islamic history signifying divisive outsiders.
The Islamic State has made more inroads in areas where either it or the government has stamped out alternative insurgent forces. That was the case in Palmyra, the first city the group took directly from government forces, which had crushed a rebellion there in 2012.
One resident of Palmyra who opposes both the Islamic State and the government said it was trying to persuade townspeople to view it as a liberator.
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“For the last seven years, they have been winning hearts and minds in Iraq,” he said, using only a nickname, Dahham, for safety reasons. “This is the same approach they are using in Palmyra, and nobody stood against them.”
Rami Jarrah, an antigovernment activist who also opposes ISIS, said the group scored a victory by destroying Tadmur Prison. The site has powerful resonance across the spectrum of opponents of Mr. Assad, from secular communists to Sunni Islamists accused of taking part in a Muslim Brotherhood insurgency that was crushed in the 1980s.
Syrians who lost loved ones to abuses by the governments of Mr. Assad and his father, Hafez al-Assad, may now feel that while global powers ignored their pleas for help, “justice has been met at the hands of extremist militants,” Mr. Jarrah wrote on Facebook.
With the recent gains by the Islamic State, Washington is tinkering with tactics and weapons. Anti-tank missiles are on their way to Iraq, to destroy American tanks that the group took from fleeing Iraqi soldiers.
But United States military support has its limits, given the Iraqi Army’s weakness, which in turn reflects the divisions that have made Iraqi national unity elusive.
“ISIS can only really be defeated by the Sunnis in Iraq,” said Emma Sky, a Briton who was a political adviser to the American military in Iraq. That is what happened in 2006, when the American military established the Awakening program, paying Sunni tribes to switch sides and fight Al Qaeda in Iraq and the greater Sunni insurgency.
The United States, unable to act unilaterally as it did in 2006, is now pushing the Iraqi government to arm Sunni tribes to fight the Islamic State, but little has happened in the face of objections by harder-line Shiite leaders. There is also a reluctance among Sunnis to fight ISIS, because many do not see being governed by Baghdad as something worth fighting for.
“We need assurances that those who are fighting against Daesh will have rights and be treated like Iraqis,” said Osama Nujaifi, a vice president of Iraq and a Sunni.
Some Iraqi Sunnis, having seen what ultimately happened with the Awakening, take a once bitten, twice shy approach now. Envisioned as a long-term effort to empower the Sunni community, the Awakening collapsed because the Shiite-led Iraqi government never fulfilled promises of full- time jobs to the Awakening fighters.
Sheikh Wissam al-Hardan, a Sunni from Anbar Province who is loyal to the government, said that Sunni fighters still on the payroll of what remains of the Awakening groups had not been paid in 15 months.
“There is no reconciliation between the government and the Sunnis,” he said. “The government considers them as if they are all ISIS.”