isis-iraq-war-crimes.siPerhaps history will mark this as the week that President Obama recognized that evil unimpeded will devour everything before it. We say perhaps because with this President you never know.

President Obama said Thursday night he authorized limited air strikes against the Islamic State in Iraq and al-Sham (ISIS) to stop the Sunni jihadists from carrying out a genocide in northern Iraq. What he didn’t do, but should, is make a larger U.S. military commitment against ISIS both to avert a humanitarian catastrophe and protect American security interests.

After routing Iraq’s army from Mosul and most of northern Iraq in June, ISIS has grown as a military force. It captured significant war materiel, including armored U.S. Humvees, and has attracted hardened jihadist fighters from Syria and elsewhere. In addition to the sums it looted from Mosul’s banks, the group has the potential to gain access to revenue from oil fields in northern Iraq.

ISIS is also threatening the obliteration of the Christian population in northern Iraq. An assault by ISIS’s forces in northern Nineveh province has emptied towns of their Christian populations. Some 40,000 Yazidis, a minority who have lived in Iraq for millennia, are now isolated with little food or water on Mount Sinjar. ISIS controls all roads out and has proven it will have no compunction to slaughter those who try to flee.

When ISIS captured Mosul, it often painted an “N” on the houses of Christians, denoting they are of Nazareth, where Jesus grew up. The Christians’ confiscated properties have been given to Muslims. Ancient Christian churches have been razed. The self-proclaimed “Islamic State” is a barbaric, pre-modern movement whose goal is to expand its dominion with mass killings. Unresisted, it will not stop.

Despite ISIS’s obvious threat to the viability of Iraq—it now also threatens Kurdistan in the north—the Obama Administration to this point has done nothing significant for more than two months to help the Iraqis fight back. Instead, it has insisted that the Iraqis in Baghdad first depose Prime Minister Nouri al-Maliki and form a more “inclusionary” government.

Under current circumstances, this policy defines “beside the point.” It has become a pretext for not acting, as if ISIS will pause while Baghdad organizes itself in a way that meets Mr. Obama’s standards. It is past time for the U.S. to intervene.

Intervention would mean primarily providing air support for the Iraqi army that is now hunkered down protecting Baghdad and the Shiite holy places in the south. It would mean in particular reinforcing Kurdistan’s peshmerga fighters with much-needed weapons and air support, including U.S.-manned Apache helicopters. With air support, these Iraqi forces on the ground can fight back. Without it, the Kurds could soon be overwhelmed by ISIS’s gathering strength.

In the corridors of realpolitik, it is always possible that even events that shock the conscience in seemingly faraway places will be seen as insufficient to justify U.S. involvement, especially after the opinion-poll fatigue of wars in Iraq and Afghanistan.

What is happening in Iraq, however, won’t remain faraway. The men driving their Islamic State have no intention of settling down in dusty towns to a contented, Shariah-led home life. The Islamic State is a dynamic, messianic, outward-moving force.

If Kurdistan falls under its control, ISIS would move to consolidate its power in Syria and destabilize Jordan. The Islamic State’s base of power and financing assured, it would export skilled jihadists to destabilize political and economic life throughout the Middle East. It is a fantasy to believe the killers on view this week in Nineveh province will stay out of Europe or the U.S.

For President Obama, any decision to reinforce the Iraqis or Kurds will be painful. On Thursday night he still took credit for removing all U.S. forces from Iraq in 2011. He was at pains to reassure Americans that the U.S. will not fight another war in Iraq even as he committed U.S. air power to the war that continues in Iraq.

It’s clear now that his decision to withdraw all U.S. troops in 2011 was a strategic and increasingly a moral disaster. The President—which is to say the United States—bears responsibility now for the humanitarian catastrophe occurring in Iraq, just as it did for the mass flight of Vietnam’s boat people, some two million, after the Communist triumph in the 1970s.

As always, there are risks to a renewed U.S. presence. Surely, though, that risk is many times greater today because the U.S. failed to field a residual force three years ago that would have prevented the march of jihadists in Iraq. Absent greater U.S. military intervention now, the march will continue.