While deep, ancient religious and sectarian divisions, combined with the failure to create law-based democratic institutions, are the root causes of the strife that continues to spread across the Middle East, terrible foreign policy decisions by successive U.S. administrations have helped ignite the unrest.

Under President George W. Bush, the United States invaded Iraq on the premise of protecting national security from the weapons of mass destruction being allegedly amassed by Iraqi dictator Saddam Hussein. When our military found no such weapons, focus turned to the nobleness of having toppled a tyrant.

Yet he was a tyrant who, however terrible, had kept the sectarian factions in that divided, post-colonial creation of a country, in check. Those factions are again unleashed as Iraq confronts civil war. It is hard to see what the U.S. gained with the cost of 4,500 soldier lives and a $1 trillion investment.

Bush successor, President Barack Obama, pursued an agenda more political than tactical. Fulfilling a campaign promise, President Obama withdrew all U.S. forces, when strategic considerations called for a small contingent to remain behind to continue training the Iraqi Army. Combined with the Bush administration's policy to break up the professional army that existed under Hussein, it has left the Iraq militarily ill-equipped to deal with the threat it faces.

In recent days, militants aligned with the jihadist, Sunni-based Islamic State of Iraq and Syria (ISIS) have swept from the Syrian border and through northwestern Iraq to take control of large swaths of territory and overrun Mosul, Iraq's second largest city. Iraqi forces wilted, soldiers stripping out of their uniforms and retreating, leaving their arms and equipment to the rebels.

Meanwhile, Kurdish forces in the north filled the void and moved into the oil-production city of Kirkuk, apparently pressing their long-held desire for a Kurdish state.

From the start, both the Bush and Obama administrations should have done far more to pressure the Shiite-led government of Prime Minister Nuri Kamal al-Maliki to embrace diversity. Instead, Mr. Maliki has persecuted Sunni political leaders and refused to yield any degree of autonomy to regions dominated by Sunnis. Now he cries for U.S. help.

Fearful of seeding a Maliki dictatorship, political leaders representing minority groups in the country have so far failed to support the emergency powers the prime minister seeks to respond to the crisis. Mr. Maliki may well act above the law without their approval.

The fighting in Iraq must be seen in the larger context of a Sunni against Shiite sectarian conflict that is spilling across the Middle East. Described as an offshoot of al-Qaida, but reportedly even more ruthless and radicalized, the ISIS force emerged amidst the chaos in neighboring Syria, where a long civil war has failed to oust President Assad.

Again there is seen the hand of U.S. foreign policy. President Obama seemed to encourage the rebellion and signal U.S. backing of rebel forces when, in August 2011, he famously declared, "For the sake of the Syrian people, the time has come for President Assad to step aside."

But the administration has done little since to support the rebellion, missing an opportunity to back with arms and training those forces more likely to back democratic institutions in the wake of the ouster of Assad. Instead, the most zealous, revenge-minded forces, such as ISIS, have gained the greatest territory in northern Syria.

Americans may be tempted to turn a blind eye to the whole affair, but that is not a reasonable option. ISIS could well create a terrorist haven out of the adjoining territories it controls in Syria and Iraq, boosting the terror threat to the U.S. and Europe.

The sectarian contagion could also continue to spread. NATO-member Turkey, which borders on Iraq and Syria, called an emergency meeting of NATO ambassadors. Out of small wars, big wars can come.

Bad foreign policy choices have left no good choices now. U.S. air strikes against ISIS forces would further anger Sunnis and cast the U.S. as Mr. Maliki's backer, but there may be no other way of stopping the advance. The better choice is an international response, but that is always a difficult proposition.

This is a dangerous mess. Having inherited and then contributed to it, the Obama administration will be hard pressed to easily clean it up.