The cold-blooded beheading of American James Foley was the Islamic State group's savage way of sending a message to the United States to stop using its air power against them in Iraq.

Let's send one back to the brutal killers.

Until now, the U.S. military has judiciously limited its bombing in Iraq to clear a path to safety for the Yazidi, a religious minority the Islamic State group targeted for genocide, to protect Americans and to provide cover for Iraqi forces fighting to retake territory from the radical invaders.

This slaughter of Foley, who was working as a photojournalist in Syria when he was captured two years ago, should expand that mission to include punishing the Islamic State group by hammering its fighters from the air. U.S. airstrikes appeared to intensify on Wednesday. They should be relentless. The U.S. military should do all it can to avoid civilian casualties -- although the hard reality is some will be killed. But the Islamic State group is a legitimate military target.

President Barack Obama on Wednesday likened the group to "a cancer" that must be extracted by governments and people across the Middle East so that it doesn't spread.

He's right. But Americans shouldn't think for a minute the terrorist group can be defeated from the air. Obama has been resolute that the United States will not be drawn back into a ground war in the region. That's still the right call, despite the provocation of Foley's murder and the soul-searing plight of Steven Sotloff, another captive American journalist.

It's up to the Iraqi forces to wage war on the ground to retake their country, mile by hard-fought mile and for the nation's political leaders to form a coalition government.

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The politics of the region are ridiculously tangled. The Islamic State group that we're bombing in Iraq includes some of the rebels we had armed in Syria where they are fighting to depose dictator Bashar Assad.

Foley was believed to have been held by Syrian forces loyal to Assad but a rescue mission last month failed to find him and other hostages. Speculation was that forces loyal to Assad handed Foley over to the terrorist group, a death sentence. Execution is the Islamic State group's signature whether the victims are Muslims, Christians or Yazidis. Any U.S. retaliation would weaken Assad's enemy.

Despite these muddled alliances in the region, Foley's bloodcurdling execution must not go unanswered.