Former President George W. Bush, who before leaving office warned against withdrawing U.S. troops from Iraq too early, told Fox News that America has since learned the "lesson" that Iraqis are not yet capable of providing for their own security.

The former president, who spoke with Fox News' Brian Kilmeade, reacted to President Obama's decision to launch airstrikes in Iraq -- and now Syria -- in a new war against the Islamic State.

Asked if the Iraqi people failed to seize the opportunity to build a new country, Bush stressed: "It's not over."

But he said the U.S. military intervention shows the Iraqi people still need help.

"The Iraqi people obviously are going to have to make a decision as to whether or not they want to live in peace," the former president said. "They're not ready to do it on their own, and that's the lesson we've learned recently."

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Bush, in 2007, had delivered a prescient warning about what might happen if U.S. troops withdrew too early. He said at the time this would risk "mass killings on a horrific scale" and potentially draw U.S. troops back into the country.

Those predictions have since come true.

Asked Thursday how he knew, Bush said: "I know the nature of the enemy."

"Anybody who kills 3,000 innocents and beheads people because of their religion or because of their point of view is dangerous," Bush said.

He said the long-term strategy to confront this threat and foster democracy "takes time," and lamented what he described as "impatience" with that process.

"Americans have got to understand that the lesson of 9/11 is still important today as it was right after 9/11, and that is the human condition elsewhere matters to our national security," he said.

Bush said he agreed with Obama's military advisers that the U.S. should have left a residual force when it pulled out at the end of 2011.

But he said he's not going to "second-guess our president -- I understand how tough the job is."