If the United States is serious about thoroughly defeating ISIS, it must -- somehow, some way -- go through Syria.
But how? And in what way?
Those are the big questions now, as President Barack Obama weighs what to do inside the war-ravaged nation where ISIS leaders are based and where the Islamist terror group rose to prominence.
Obama ceded Thursday that "we don't have a strategy yet" for what to do about ISIS inside Syria, with a senior administration official adding that a decision is "a week or so" away.
There are certainly options, but none is clear-cut.
"There is no such thing as a no-risk strategy," said Rep. Adam Smith, D-Washington. "It's a matter of taking the right risk and balancing that risk to make the right choice."
Here's a look at some possibilities, including why and how they could and could not work:
1) Ground forces
In other words, go all in.
When terrorists attacked on September 11, 2001, the United States showed its willingness to use its full military might when it attacked Afghanistan -- a campaign that dragged on for years and killed more than 2,300 American troops.
That steep cost, both human and financial, is the big reason this is very, very unlikely to happen again in Syria.
No U.S. officials have suggested troops on the ground. The fact none were sent to fight in Iraq -- a country where the U.S. has deeper ties and a government it works with -- is further indication there will be no U.S. ground invasion of Syria anytime soon.
There could be smaller-scale, targeted operations, though. After all, Pentagon spokesman Rear Adm. John Kirby told CNN that, this summer, elite U.S. commandos from units like Delta Force and Navy SEAL Team 6 went into Syria and tried to rescue American journalist James Foley and others held by Islamic militants.
They didn't find hostages, but who's to say U.S. Special Operations Forces couldn't conduct more such missions to save others or for some other purpose? Then there's the possibility American troops could go into Syria to help with the targeting of U.S. airstrikes.
Of course, all bets are off if ISIS pulls an al Qaeda and strikes inside the United States.
That hasn't happened yet, though some experts believe that such an attack from ISIS -- which, upon beheading Foley, warned other U.S. citizens could be next -- might be a matter of time.
Obama gave the go-ahead to pound ISIS forces in Iraq from the air. Why not do the same in Syria, a country the President himself ceded Thursday has become a "safe haven" for the terror group?
Except it's not that simple.
It starts with the fact that Syria is a mess. Three years of civil war have torn apart the country, spurring the emergence of rebel groups fighting to oust President Bashar al-Assad and sometimes against each other. About the only thing al-Assad and rebel groups have in common is that ISIS is their enemy.
Obama is pushing to get al-Assad too, but they're on the same side when it comes to ISIS. Yet U.S. officials insist this shared cause doesn't mean they'll coordinate any military action with al-Assad's government, even if Syrian officials are demanding it.
It's one thing to anger Syria even more. It's another thing to anger its allies Iran and Russia. Russia, which is already at odds with the West over Ukraine, could block any U.N. Security Council effort to give a seal of approval to international strikes.
All of this brings many questions: Does the United States really want to conduct a military campaign in a country without a government that is stable and that it trusts? Can it count on opposition factions it supports to provide any long-term stability? And does it know that airstrikes will wipe out ISIS in Syria rather than stalling the group?
Absent a fortune-teller, it's hard to tell how any military action would turn out.
Still, if Obama decides that's the way to go now, White House spokesman Josh Earnest stressed, more needs to be done politically, diplomatically and economically long-term to keep ISIS down.