When President Obama went on TV last week to outline his response to terrorist advances in Iraq, he missed a chance to do something essential: convey how serious the threat is to the Mideast — and to us.

The practical steps he proposed made sense in the short run (although they should have been taken at least a year earlier): Increase U.S. intelligence surveillance of Iraq and Syria; send up to 300 more U.S. military advisers to Iraq to learn what’s really going on; make an intense diplomatic effort to head off a wider sectarian holy war; and position military assets such as ships in the region in case the terrorists move on Baghdad. The president ruled out sending any combat troops — ever — and held off, for now, on air strikes or drone attacks.

But Obama failed to make clear to skeptical Americans why they should care about Iraq’s current troubles, or why this crisis is so terrifying to those who know the region. The U.S. public needs to understand why this challenge is so enormous and why U.S. officials must focus, again, on Iraq.

So here goes:

The current crisis was sparked this month when an al-Qaida offshoot known as the Islamic State of Iraq and Syria (ISIS) took control of Iraq’s second-largest city, Mosul. As the Iraqi army collapsed, ISIS forces moved rapidly toward Baghdad.

The group was well known to U.S. officials. Born in Iraq, then decimated by U.S. troops in the 2000s, it reconstituted itself in war-torn Syria last year and conquered the northeastern part of that country. It moved into Fallujah in western Iraq early this year.

But its seizure of one-third of Iraq this month marks the first time a radical jihadi group has taken control of a nation-sized swath of territory, erasing borders that had existed since the early 20th century. ISIS has pledged to restore the Islamic caliphate that ended with the fall of the Ottoman Empire in 1924. In areas of Syria it controls, ISIS has set up courts and schools and taken control of other government services.

Not only does the group control land, it also has the financial and military resources of a state.

Once reliant on cash from private donations from rich gulf sheiks, it increased its wealth through taking control of Syrian oil fields, illegally selling valuable antiquities, and extortion. Now it has added about $400 million seized from Iraqi banks in Mosul. And it has an arsenal of heavy U.S. weaponry taken from Iraqi army depots it captured this month.

“This is the biggest challenge to the United States since 9/11,” says Jim Jeffrey, former U.S. ambassador to Iraq, in comments typical of those I’ve heard from many experts. “This is the largest concentration of al-Qaida anywhere, and they are the nastiest.” In fact, ISIS was so extraordinarily violent in Syria that core al-Qaida disavowed the group, worried it would alienate the locals.

Some experts argue that the ISIS threat is overrated because it has only 7,000 to 10,000 members and because its advance on Baghdad has stalled.

They also say ISIS gains in Iraq depend on cooperation from Sunni tribes that have been alienated by the avidly sectarian rule of Shiite Prime Minister Nouri al-Maliki. They predict these tribes will ultimately evict the jihadis. Thus, they claim, no American focus on Iraq is necessary.

Such arguments gloss over the depth of the ISIS threat to U.S. interests, and to Iraq.

First, ISIS has already attracted thousands of foreign fighters, including at least several hundred Europeans and dozens of Americans, one of whom died as a suicide bomber in Syria. Top U.S. and European intelligence officials worry that some of these trained fighters will be dispatched back to their homelands. Moreover, even if ISIS halts at Baghdad’s gates, the group’s astonishing Iraqi successes will attract more foreign fighters, including Westerners.

Second, Maliki has been able to stall ISIS only by relying on extensive help from radical Shiite militias and from Iranian Revolutionary Guard officers. Unless the Iraqi leader can be persuaded to step down or form a national unity government with Sunnis, he is likely to provoke a wider sectarian war in the region that could spread to Lebanon, Jordan, and the gulf. This would threaten untold civilian lives, not to mention U.S. allies and oil.

Moreover, if Maliki and Iran hold to their sectarian ways, Iraq’s Sunni tribal leaders will be reluctant to break with ISIS. Radical Shiite militias and Sunni jihadis will stoke the killing. ISIS will revel in a regional holy war that pits Sunnis against Shia, Sunni Saudi Arabia against Shiite Iran.

Only intense, U.S.-led regional diplomacy offers a slight chance of averting this grim scenario, by persuading Iran, Saudi Arabia, and Iraqi factions that sectarianism threatens them all (Iranian officials appear split on this issue). Only U.S. diplomacy, led by Obama, might lead to an Iraqi government of national unity. Only then might there be a chance to roll back ISIS inside Iraq.

And if U.S. diplomacy fails, the White House must be positioned to prevent ISIS from threatening U.S. interests, using drones if necessary. But long before then, Obama needs to explain to Americans why Iraq still matters to them.