President Obama previewed his plan for dealing with the Islamic State by comparing it to counterterrorism operations in recent years and said it would not be an invasion akin to the ground war in Iraq.
As he prepares to lay out the details in a speech to the nation Wednesday night, several key factors are likely to determine the success or failure of any military mission.
1. How much can be accomplished with air power alone?
The U.S. has already carried out some 150 airstrikes in northern and western Iraq in a limited campaign that has halted the Islamic State advance and driven it back in key places like the Mosul dam.
Obama will be looking to build on this success and is expected to broaden the air campaign in Iraq while possibly extending it to Syria. In both countries, Islamic State fighters are vulnerable and exposed when moving over flat desert terrain to and from the cities they control along the Tigris and Euphrates rivers.
But air power has its limits. Bombing alone is unlikely to chase the Islamic State out of big cities like Mosul, and the president has emphatically ruled out sending in ground troops.
Robert Scales, a retired major general and former commandant of the U.S. Army War College, acknowledges that the U.S. public won't support a major land war. But he and others are already arguing for the use of small teams of U.S. Special Operations forces.
"The Islamic State cannot be defeated by diplomacy, sanctions, coalitions or political maneuverings," writes Scales. "The only sure means for defeating the group is with a renewed, expanded and overwhelming legion of capable special fighters who have learned through painful trial and error how to do the job."
2. Can the U.S. find effective allies on the ground?
The crisis in Iraq is largely a result of the weak military that dropped its weapons and fled when the Islamic State came rampaging into the country. The U.S. trained the Iraqi military for years, but its quality declined after the U.S. left in 2011 and many senior officers were sacked. It's not clear how Iraq will be able to turn its military around in the short term.
"Before the United States assumes a broader imperial mission, it should demand more of those in the region that are most directly threatened by ISIS," writes Brian Jenkins, a counterterrorism expert.
"Iraq's army has not won a single battle and cries for American help. If Iraq cannot defend itself with nearly 300,000 men under arms, can America protect it?" he adds.
Things are a bit brighter in northern Iraq, where the U.S. is allied with the Kurdish peshmerga militias, who are considered a competent fighting force and have been working with the U.S. to retake areas bombed by the Americans. This would be the model the U.S. would like to replicate, but that may prove a tall task, especially in Syria.
The U.S. has been talking about helping "moderate" Syrian rebels for the past couple years, but has provided only minimal assistance so far. In Syria's brutal war, the moderates are getting increasingly hard to find and their presence on the battlefield has been steadily shrinking as the war grinds on.
3. Will there be a strong political coalition that includes European and Arab states?
Obama said nine Western nations and Turkey have signed on to counter the Islamic State. Some European states are already sending weapons to Iraq, but there's no indication so far that they will take part in airstrikes or other military missions.
Another complication is that members of the coalition may have different agendas. For European countries, the main concern may be preventing European jihadists in the Middle East from returning home to carry out attacks. Aside from Turkey, none of the countries bordering Iraq or Syria have joined the coalition.
Right now, the nascent coalition is united by an opposition to the Islamic State and the potential havoc it could wreak. But it could prove challenging for the U.S. to maintain broad international support over time.
4. Are Americans prepared for another lengthy military campaign?
Sixty-five percent of Americans now support airstrikes against Syria, according to a Washington Post-ABC News poll. That's more than double the number that felt that way a year ago when the president threatened to bomb Syria after it used chemical weapons.
The recent beheadings of two American journalists by the Islamic State provoked outrage and contributed to this surge in support for military action. But that support could evaporate swiftly if things get messy.
The U.S. is still winding down its longest war ever — the 13-year conflict in Afghanistan — and the administration and the military are already warning that the operation against the Islamic State could last for years.
Also, the goal has been fuzzy. After saying he didn't have a strategy in Syria, the president is now saying the U.S. will seek to "degrade and destroy" the Islamic State. As the U.S. found with battles against al-Qaida, the Taliban and others, that's a recipe for an open-ended mission.
Foreign policy commentator Peter Beinart describes Obama as a "fierce minimalist" who is willing to use force and take risks against specific terrorism threats. The leading example is the raid that killed Osama bin Laden.
But Obama has resisted broader campaigns throughout his presidency.
"The core of his military effort in Iraq and Syria, and throughout the greater Middle East, is narrow but aggressive anti-terrorism," Beinart writes in The Atlantic.
5. What will the unintended consequences be?
Even if a U.S. military effort goes well, the outcomes could be mixed. For example, assume the U.S. greatly weakens the Islamic State in Syria. The most likely beneficiary would be President Bashar Assad's regime — the same one the president was planning to bomb last year.
Also, Iran will be cheering for a successful U.S. military campaign. Iran's Shiite leadership loathes the Sunni extremism of the Islamic State, which is currently threatening the governments of Iraq and Syria, both of which are friendly to Iran.
At the moment, the Islamic State is preoccupied with taking territory in Iraq and Syria and building its self-proclaimed caliphate, or Islamic nation, according to many analysts. But if it comes under sustained U.S. attack, the Islamic State could place much greater emphasis on lashing out at U.S. targets.
"Before the United States engages in what could be a long and messy military campaign, it might cool the alarmist and partisan rhetoric and coolly examine the threat the Islamic State poses to America's national security. It does not surpass every threat we have seen," according to Brian Jenkins, the counterterrorism expert.
Greg Myre is the international editor of NPR.org. You can follow him @gregmyre1.