US President Barack Obama makes a statement to the press on the death of American journalist James Foley at Martha's Vineyard, Massachusetts August 20, 2014. The United States has carried out more air strikes in Iraq, a senior US defense official said, as Islamic militants threaten to execute a second US journalist. (Nicholas Kamm/AFP/Getty Images)

WITH EACH day, the barbarism of the Islamist extremists terrorizing Syria and Iraq becomes more evident — as does the need for the United States and its allies to act more vigorously to block their rise.

On Tuesday the group that calls itself the Islamic State released a video of the beheading of American journalist James Foley . Mr. Foley, as President Obama said Wednesday, “reported from difficult and dangerous places. . . . [He] courageously told the stories of his fellow human beings.” One reason there has been so little outcry as more than 150,000 Syrians have been killed and millions rendered homeless is that reporting on that nation’s brutal war is so dangerous. Those like Mr. Foley, who risk everything to bear witness, deserve our admiration, and his killers deserve our contempt.

Mr. Foley is one among thousands of victims murdered by the Islamic State as it has conquered territory in Syria and Iraq. Mr. Obama summed up its record all too well. “They have rampaged across cities and villages killing innocent, unarmed civilians in cowardly acts of violence,” the president said. “They abduct women and children and subject them to torture and rape and slavery. They have murdered Muslims, both Sunni and Shia, by the thousands. They target Christians and religious minorities, driving them from their homes, murdering them when they can, for no other reason than they practice a different ­religion.”

In recent days, the Obama administration has made some progress in blocking the progress of this terrorist group. U.S. air raids have helped Iraqi and Kurdish forces recapture some lost territory. U.S. pressure may have hastened the appointment of a new prime minister in Iraq, who, it is hoped, will work across sectarian lines better than his predecessor and so be better positioned to rally his country to defeat the so-called caliphate of the Islamic State. Mr. Obama has emphasized the importance of enlisting the Iraqi army and other local players in confronting this scourge, a goal with which we wholeheartedly concur.

But urging others into the fray will not be sufficient. Nor is it wise to assume that the Islamic State will collapse under the weight of its cruelty. “People like this ultimately fail,” Mr. Obama said Wednesday. “They fail because the future is won by those who build and not destroy.” That may be so. But history provides too many examples of destroyers who hold power for long stretches of time and do not lose it until they are dislodged by “builders” who are finally roused to action.

For three years the United States stood aside as the Islamist extremists built up their strength inside Syria. Washington was surprised in June when they burst into Iraq, captured Mosul and threatened Baghdad and surprised again this month when they threatened Kurdistan. Now, according to most accounts, they are consolidating their hold inside a large swath of territory spanning the two nations even as they fight to expand. They are training hundreds of foreign terrorists, including from Europe and the United States, who could easily slip back into their home countries with malign intent. They proudly proclaim their enmity to America. America needs a genuine strategy in response.