The U.S. Navy released footage taken from inside the cockpit of an F/A-18F Super Hornet, assigned to the Fighting Black Lions of Strike Fighter Squadron (VFA) 213, as it takes off from the USS George H.W. Bush in the Arabian Gulf. (U.S. Navy via YouTube)

Senior U.S. officials describe the threat posed by the Islamic State in chilling terms, but they have mounted a decidedly modest military campaign to check its advance through northern Iraq.

The radical Islamist organization has attracted more fighters, controls more territory and has access to a larger stream of money than al-Qaeda did before the attacks of Sept. 11, 2001, according to U.S. officials and terrorism experts. Its refusal to rein in its brand of rampant violence accounts in part for its break from the better-known terrorist group.

“This is serious business,” Secretary of State John F. Kerry told reporters earlier this week. “I think the world is beginning to come to grips with the degree to which this is unacceptable.”

So far, though, the Obama administration’s response to the group’s blitzkrieg through northern Iraq has been defined primarily by the limits it has placed on the U.S. military’s intervention.

The disconnect between the unnerving assessments of the Islamic State and the apparent lack of urgency in confronting it reflects a mix of political and military constraints. Among them are no clear military strategy for reversing the group’s recent territorial gains, a war-weariness that pervades the Obama administration and the country, and significant uncertainty about the extent to which the Islamic State is prepared to morph from a regional force into a transnational terrorist threat that could target Europe and the United States.

The U.S. military’s campaign against the Islamic State has focused on protecting U.S. citizens in Baghdad and Irbil and delivering aid to thousands of Yazidi refugees who had been trapped on Mount Sinjar in northern Iraq and appeared Wednesday to be making their way down to safety.

But the ongoing U.S. airstrikes are equally notable for what they have not tried to do. U.S. military officials have emphasized that the strikes are not designed to reverse the gains Sunni extremist fighters have made.

“We’ve had a very temporary effect,” Lt. Gen. William Mayville, a senior Army officer on the Joint Staff, told reporters this week.

The limited nature of the airstrikes has drawn criticism from more hawkish Republicans and some former U.S. military officials who have said that the Obama administration is squandering an opportunity to deliver a crippling blow against the insurgents.

“Time is of the essence,” said Adm. James Stavridis, a former supreme allied commander of NATO and now dean of the Fletcher School at Tufts University. The longer the airstrikes drag on, the more time Islamic State fighters will have to learn how to survive them. “Without a fast and serious response, including Special Operations forces on the ground, the chances of reversing IS gains or even breaking their evident momentum is very low,” he said.

Pentagon officials do not dispute that assessment, but they are deeply skeptical about the effectiveness of U.S. air power without credible Iraqi partners on the ground. “My major impression is that they just don’t know what to do right now,” said Celeste Ward Gventer, a former senior Pentagon official and adviser to the U.S. military in Iraq.

The current generation of Islamic State fighters seems to be better armed and trained than the largely homegrown insurgents who fought the U.S. military in the years after the American invasion of Iraq, military officials said. The United States has far fewer warplanes operating over Iraq than it did during the height of the Iraq war.

The U.S. military released video of targeted airstrikes in northern Iraq over the weekend. (YouTube: CENTCOM)

“We had a vastly larger Air Force in Iraq from 2003-2009, and it didn’t defeat the insurgency,” said Stephen Biddle, a professor at George Washington University and frequent adviser to the U.S. military. “If we couldn’t do it then, it is hard to imagine a much smaller effort will be more effective today.”

The United States could make more inroads with a larger ground force, but some military analysts have said that the Obama administration would have to be willing to deploy several thousand troops to push militants out of cities such as Fallujah, Mosul and Tikrit. Such a force would include counterterrorism commandos, intelligence specialists, medical personnel and a large advisory presence to work with Iraqi soldiers and help persuade Sunni tribal fighters to take up arms against the insurgents, said retired Col. Derek Harvey, a former top U.S. official in Iraq and at U.S. Central Command.

“I had been saying we’d need as many as 6,000 [troops],” he said. “But it’s probably close to 8,000.”

President Obama, who campaigned on ending the wars in Iraq and Afghanistan, has repeatedly said that a U.S. presence of that size in Iraq isn’t under consideration. “American combat troops will not be returning to fight in Iraq, because there’s no American military solution to the larger crisis in Iraq,” he said.

With that in mind, the Obama administration has held off on more aggressive intervention plans, pledging in recent days to expand U.S. military involvement if the Baghdad government can show progress on including Sunnis and Kurds.

The approach represents a significant gamble by the administration that it can afford to wait to carry out a broader assault against the Islamic State and make it contingent on the precarious prospect of political reconciliation in Iraq. Regional experts said getting disenfranchised Sunnis to take up arms against Islamic State fighters on behalf of the Shiite-dominated government in Baghdad could take years.

“I don’t think we have time,” said Nada Bakos, a former counterterrorism analyst with the CIA who served extensively in Iraq. “They are a problem for us already.”

In June, authorities in Germany and Spain arrested 11 alleged members of the organization, including a former detainee at Guantanamo Bay, Cuba, in connection with efforts to direct fighters to Syria. The Islamic State’s extensive efforts to recruit Westerners, as well as indications that a dozen or more Americans are among its ranks, mean that it already is a threat to the United States and its allies, Bakos said.

The administration’s patience is based to a large degree on its assessment that the Islamic State is for now consumed by its regional ambitions, and an implied confidence that U.S. spy agencies will be in position to detect when the organization crosses the threshold from regional problem to transnational terrorism threat.

“At this point, [the Islamic State’s] efforts are largely limited to Syria and Iraq,” a senior U.S. intelligence official said. “However, their own spokesperson has talked about fighting the United States.”

Other analysts said that the ruthlessness and fighting prowess that the Islamic State has employed on battlefields in Syria and Iraq are distinct from the kinds of organization and planning associated with al-Qaeda’s terrorist plots.

“What we are seeing in Iraq is a very capable insurgent ground force,” said Daniel Benjamin, a former top State Department counterterrorism official. “That’s not the same as a very capable international terrorist operation.”

Most terrorism experts said the threat posed by the Islamic State is likely to increase as fighters with Western passports return home.

Beyond its battles with the Iraqi and Syrian governments, the Islamic State also is facing pressure to govern and provide basic services in the vast territory it now controls. All of these factors could delay its evolution in to an international terrorist organization and buy the Obama administration time for a political solution to the unrest in Iraq.

“Bottom line: We are likely to have a confrontation with IS in the future,” Benjamin said. “The threat will almost certainly grow.”