The White House wants to find out and is deploying as many as 300 military advisers to Iraq to assess the might of the Islamic State in Iraq and Syria.
The jihadi group's rash battlefield successes make it look extremely fierce.
ISIS militants have surged from the border with Syria to blitz major cities in Iraq's northern Sunni region, taking Tal Afar and Mosul, then moving quickly south. Hundreds of thousands of civilians fled from their path, creating a new refugee crisis.
They have advanced on Baquba, just north of Baghdad, and are threatening to attack the capital.
The Obama administration has said there will be no more American boots on the ground after the drawdown of all American troops -- tens of thousands of them.
It's up to the advisers to help Iraqi security forces vanquish ISIS on their own.
Washington officials have said little about what they'll actually be doing, and expert opinions on that and on whether they should be in Iraq at all are split.
Who are they?
They are high-ranking officers. They are Navy SEALs and Army Rangers, said retired Marine Sgt. Adam Banotai.
Banotai, who scrapped through the brutal battle for Falluja during the Iraq war, thinks the term "adviser" is misplaced.
"It is political semantics," he said. "We are calling them adviser now ... instead of combat troops or boots on the ground," he said.
"They are the most elite fighters we have," he added. "So, if they aren't going to be combat troops, I'm not quite sure who the President is going to refer to as combat troops."
Where will they be?
"They'll be sitting in offices, not out on the front lines, and they'll be looking at the maps," said retired Lt. Col. Douglas Ollivant.
"So, they're sort of holed up, giving advice," he said. It would be unwise to put them on the front lines, where ISIS could capture them.
How will they work?
The plan is for the advisers to divide into small teams of about a dozen each and partner with Iraqis to form joint operation centers, senior administration officials said.
What will they do?
Their job will be to gather intelligence, in case President Barack Obama decides to pull the trigger on airstrikes, a possibility he's weighing.
The advisers will also pass on intelligence to Iraqi forces, said retired Lt. Gen. Mark Hertling, who commanded U.S. forces during operation Iraqi Freedom.
Should they be there?
That depends on whom you ask.
Ret. Col. Doug MacGregor says no. He says it's a largely meaningless gesture.
"They'll be at risk," he said. He says he thinks sending them was the least bad of a list of bad options presented to the President, who felt he had to do something.
Maj. Gen. James "Spider" Marks disagrees.
The United States has to push back against ISIS, he said.
How dangerous are the militants?
MacGregor calls them "semi-illiteral (illiterate) thugs driving around in pickup trucks with machine guns."
They won't pose a military threat to the United States, he said. "They're occupied with decapitating Shiites and trying to establish an Islamist state in vast open stretches of empty, irrelevant desert."
Picking up on MacGregor's term, Marks said that "Illiteral (illiterate) thugs" have struck the United States before and could come again.
"We can't allow that to occur any more than it is right now in Iraq," he said.
The group has shown signs of sophistication, publishing complex reports on its finances.
It has recruited fighters from multiple countries and bonded with professional ex-members of former Iraqi leader Saddam Hussein's armed forces. And Sunni tribal leaders have also joined their ranks.
ISIS also holds much of the northernmost region of Syria.
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Will the advisers' presence help?
That's yet to be seen.
Obama has said that his goal is to prevent Iraq from becoming a haven for terrorists.
Some have criticized him for not leaving a residual force in the country to prevent the type of invasion ISIS has mounted.
But Obama is quick to point out Iraq's role in that choice by not protecting U.S. troops from Iraq's justice system in the event of bloodshed.
"The Iraqi government and Prime Minister (Nuri al-Maliki) declined to provide us that immunity," Obama said.
Retired Sgt. Banotai feels iffy about Iraqi soldiers' commitment.
When the ISIS jihadis overran the northern cities, many Iraqi troops put up little resistance and fled.
"I had friends over there who had the exact opposite experience where over 50% of a unit would desert at a given time and a firefight would happen and they would drop their rifles and run."