Iraqi Revolution via AP video, File, Associated Press
President Obama needs to make good on his promise to be "vigilant … and relentless" in battling ISIL in Iraq.

President Obama struck the right tone this week in his reaction to the brutal beheading of American journalist James Foley by militants with the Islamic State of Iraq and the Levant, or ISIL.

“We will be vigilant and we will be relentless,” he said. “When people harm Americans anywhere, we do what’s necessary to see that justice is done and we act against ISIL, standing alongside others.”

It was the sort of tough rhetoric one expects from an American president. In this case, however, it left some giant, unspoken questions hanging in the air. Just exactly what is it this president intends to do? Will Obama, who has acted as if his promise to end the war in Iraq was more important than responding to any looming threats to civilization in that region, now support the type of ground-troop offensive necessary to eradicate Islamic State fighters?

If not, how does he intend to be “vigilant” and “relentless” and to “see that justice is done”?

One encouraging sign was the recent way in which Obama forced the resignation of Iraqi Prime Minister Nouri al-Maliki, by refusing to launch airstrikes against radical Islamic forces until he stepped down. That was decisive action solidly in support of administration policy.

But the resulting airstrikes, although successful in pushing Islamic State forces away from the Mosul Dam and other strategic areas, will not be enough to constitute a relentless pursuit of the enemy.

High-ranking administration officials, not known for hawkish exaggerations, have referred to the ISIL in dire terms. It poses a threat of a “dimension that the world has never seen before,” said Defense Secretary Chuck Hagel.

That’s a strong statement. It puts ISIL above Hitler or any other megalomaniacal tyrant or organization with visions of global domination in history — and those tended to require unrelenting, all-out military responses.

It’s time for President Obama to address the American people and to make the case for how he intends to back up his rhetoric. This week, he said the United States will “act against ISIL, standing alongside others.”

That is an unfortunate indication that he might consider doing nothing unless he can gather widespread international support. In any case, he needs to begin building that support quickly.

There is evidence ISIL poses threats to much of the world, including other regional forces such as Syria. The president talked of a “common effort” across the Middle East. That may require uneasy alliances with nations the United States normally considers enemies.

The United States might not have found itself in this position if the Obama administration had shown greater leadership against threats in the world, ranging from Russia’s incursions into Ukraine to attacks against U.S. diplomats in Libya.

Forces at war with civilization are quick to take advantage of a perceived lack of resolve in Washington. As columnist Michael Gerson wrote this week, “If America does not lead the global war on terrorism, the war will not be led.”

Unfortunately, that is a responsibility that can’t be shirked.