"We've got to win and stop these guys."

That was Sen. Lindsey Graham, Republican of South Carolina, criticizing the Obama administration on CNN as Islamic State fighters (also known as ISIS) gobbled up chunks of Syria and Iraq. His sense of alarm is clearly justified.

The jihadist militants pose a serious threat to a range of American interests, from the security of the homeland to the stability of critical allies like Israel and Turkey.

Even some Democrats were upset with President Obama for not matching Graham's urgency when he told a press conference "we don't have a strategy yet" for halting ISIS' ambitions. Sen. Dianne Feinstein, a California Democrat, spoke for many when she suggested on NBC that the president was "maybe, in this instance, too cautious" in his response.

But calling for the defeat of ISIS is the easy part. We know who we're against in the Middle East. Knowing who we are for is a much more difficult problem.

That question cannot be avoided, however. Recent history has graphically taught us in too many places -- Iraq, Libya, Egypt -- that deposing a despotic regime is no guarantee that the bad guys will be replaced by a government more friendly to human rights and our core values.

That was the profound mistake made by the Bush administration when it invaded Iraq. Administration officials firmly and naively believed that Saddam Hussein would be replaced by a democratic government that could reconcile warring tribes, sects and ethnicities who had despised each other for centuries.

It was a tragic illusion that cost thousands of American lives and billions of American dollars. But that's the memory that haunts Obama today. That's the question that keeps him so cautious. Who are we for? What forces fill the vacuum when the dictator disappears?

In a revealing interview with Thomas Friedman of the New York Times last month, the president cited the case of Libya as "an example of a lesson I had to learn that still has ramifications to this day."

Deposing Moammar Gadhafi was "absolutely ... the right thing to do," he said, because leaving him in place would have led to "more death, more disruption, more destruction."

"But what is also true," Obama continued, "is that I think we (and) our European partners underestimated the need to come in full force if you're going to do this." They underestimated how hard it would be "to rebuild societies that didn't have any civic traditions."

"So that's a lesson that I now apply every time I ask the question, 'Should we intervene militarily?'" he told Friedman. "Do we have an answer (for) the day after?"

Across the Middle East, there are no good answers to that question. In Iraq, the Shiite-led government of Nouri al-Maliki "squandered an opportunity," in Obama's words, to replace Saddam with a regime that respected the nation's minorities, particularly Kurds and Sunnis. There is some hope that a new post-Maliki government in Baghdad will be more inclusive, but the country remains far from stable.

Moreover, organizing forces to oppose ISIS presents its own risks. As the Wall Street Journal reports, the defeat of the jihadists in the town of Amirli was led by Shiite militias who answer to Muslim clerics, not any civil or military authority.

Accordingly, says the Journal, the strategy of working with these militias "could end up empowering armed groups who have worked against U.S. interests in the past and whose longer-range political goals may undermine the country's unity."

In Syria, Obama told Friedman, it has "always been a fantasy" that the rebels challenging Bashar Assad could ever mount an effective threat to his regime, so sending them arms and advisors would have been a futile gesture.

The only force that does pose a military challenge to Assad is ISIS. But partnering with the jihadists in Syria would be even more dangerous than lining up with the Shiite militias in Iraq.

Almost six years in office tells the president that "maximalist forces" are the deadly enemy of American aspirations. He cites the failure of the Maliki government and says, "That's, by the way, a broader lesson for every country: You want 100 percent, and the notion that the winner really does take all, all the spoils. Sooner or later, that government's going to break down."

So yes, Obama is a cautious leader; perhaps too cautious. But he has trouble answering two key questions: Who are we fighting for? What happens the day after the bombing ends?

So caution is justified.