WASHINGTON -- The Obama administration is grappling with how to bridge the gap between its increasingly dire assessment of the threat posed by the Islamic State group and the limited, defensive air campaign it has so far undertaken, which military officials acknowledge will not blunt the group's momentum.
For months, administration officials have been divided about the threat posed by the Islamic State as it seized parts of Syria and advanced on towns in Iraq. Now, amid new intelligence about its growing strength, a consensus is forming that the group presents an unacceptable terrorism risk to the United States and its allies.
At issue is whether President Barack Obama, elected on a platform of ending the Iraq war, will heed calls for a campaign to contain or destroy the Islamic State, an undertaking that could dominate U.S. foreign policy for the remainder of his term.
Defense Secretary Chuck Hagel said the group poses "a threat to the civilized world," while Senate Intelligence Committee Chairwoman Dianne Feinstein, D-Calif., called the Islamic State a "terrorist army" that must be defeated. But Obama has not used similar language. He has authorized a limited campaign of targeted airstrikes designed to protect refugees and American personnel in the Kurdish region -- but not take out the group's leadership or logistical hubs.
A strategy to destroy the Islamic State would not require large numbers of American ground troops, but it would amount to a significant escalation from the recent air operations, analysts say. It might also require military action in western Syria, where the group has its headquarters in the city of Ar-Raqqah.
Proponents of doing so argue that the Islamic State must be stopped because it will destabilize America's allies in the region and eventually export terror to Europe and the U.S. Critics of the idea are urging the president just as strongly not to get sucked into another Middle East war, arguing that years of American micromanagement in that region have ended in tears.
Obama himself has said the U.S. "has a strategic interest in pushing back" the Islamic State, but he has also insisted he will not send American combat troops back to war in Iraq. He has not shied away from using targeted military force in other places, such as Pakistan, Yemen and Somalia, when he decided that terrorists there threatened the U.S.
U.S. officials say thousands of Westerners -- and at least dozens of Americans -- have sought to travel to Syria to join the fighting there, and some of them have joined the Islamic State. Attorney General Eric Holder has called the mix of Westerners and Syria-based terror groups "more frightening than anything I think I've seen as attorney general."
U.S. intelligence believes that some of those Westerners are now fighting in Iraq, said a senior intelligence official who was not authorized to discuss sensitive intelligence by name and requested anonymity.
When al-Qaida in the Arabian Peninsula seized parts of southern Yemen in 2011, Obama stepped up drone strikes and used special operations to support Yemeni forces in pushing the militants out.
Smashing the Islamic State, military and intelligence analysts say, would require a sustained campaign of American airstrikes, combined with a U.S.-backed ground force of Sunni tribesmen -- the same approach that rooted al-Qaida in Iraq out of the Sunni tribal areas in 2008.
But such a campaign would be "orders of magnitudes more difficult" than Yemen because of how well-armed and well-trained Islamic State fighters are, said Peter Mansoor, a retired army colonel who helped oversee a turnaround in Iraq in 2008.
"We have a mismatch between our goals and our strategy at the present time," said Mansoor, now a professor at Ohio State. "The goal eventually is to eliminate (the Islamic State), but the president has laid out a very restrained military option which can't accomplish that goal."
Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said during a security forum in Aspen, Colorado, last month that the military is "preparing a strategy that has a series of options to present to our elected leaders on how we can initially contain, eventually disrupt, and finally defeat (the Islamic State group) over time."
Obama's GOP critics fear that the president will shy away from such a strategy because it repudiates what they say was his misguided decision to disengage from Iraq. Two years ago, the president resisted the calls of his advisers to aggressively arm moderate rebels in Syria.
"You can almost hear the angst in the voices of our military commanders connected to what they know is a fundamental mismatch" between the threat and the strategy, said Rep. Mike Pompeo, R-Kan., a former Army officer and member of the House Intelligence Committee. "President Obama absolutely is refusing to acknowledge the threat to America and respond in a way that is appropriate."
Ben Rhodes, White House deputy national security adviser, rejected that view. "We absolutely believe that (the Islamic State) poses a threat to U.S. persons and personnel," he said Wednesday. "We're focused on dealing with that threat right now in Iraq so that the terrorists cannot advance on Irbil," the Iraqi Kurdish capital.
Administration officials say the White House has been deeply divided at least since the start of 2014 over how much the Islamic State threatens Americans.
In January, when the militants overran the western Iraqi city of Fallujah, U.S. officials weighed whether to intervene. But one senior U.S. official familiar with the conversations said there were concerns that what was playing out was an internal dispute -- a revolution by Sunni tribes against the Shiite-led government and Prime Minister Nouri al-Maliki. At a result, the U.S. limited its response to providing the Iraqi army with Hellfire missiles and began tracking the militants with surveillance drones.
Since then, the number of Islamic State militants swelled from a few thousand to an estimated 15,000 die-hard members, according to two senior intelligence officials.
Many of the extremists are battle-hardened former members of Saddam Hussein's elite Republican Guard who are intimately familiar with Iraq's dusty terrain and tribal connections, say the U.S. officials, who spoke on condition of anonymity because they were not authorized to discuss the information by name.
The Islamic State, which has been disavowed by al-Qaida in a dispute over strategy, wants to strike a terrorist blow at the U.S. to assert its primacy in the jihadist movement, said Derek Harvey, a former Defense Intelligence Agency official who advises U.S. Central Command.
"They have been planning do to this for some time," he said. "We just don't know when."