After eight years of war in which training Iraqi security forces was a top mission for the Pentagon – which spent $25 billion to do it – those military forces have turned out to be a considerable disappointment from the US military’s perspective.

The Islamic State of Iraq and the Levant (ISIS) this week seized control of Mosul, one of Iraq’s largest cities, and Saddam Hussein’s hometown of Tikrit. The Islamist insurgent group’s next stop, it has proclaimed, is Baghdad. Along the way, the militants have reportedly captured US-made Humvees and trucks.

“Look, the United States has poured a lot of money into these Iraqi security forces, and we devoted a lot of training to Iraqi security forces,” President Obama said Friday. “The fact that they are not willing to stand and fight and defend their posts against admittedly hardened terrorists – but not terrorists who are overwhelming in numbers – indicates that there’s a problem.”

What has happened to this military force that US troops spent years – and US troop lives – training and equipping? And what, if anything, should the US now do to aid these forces?

“They should be doing better. There’s no doubt that they should be doing better,” says retired Lt. Gen. James Dubik, who was the commander of the US training mission in Iraq in 2007 to 2008.

The deterioration of the Iraqi security forces has not been a sudden development, however, but rather the result of a steady decline since US forces departed in December 2011. In that time, Iraqi Prime Minister Nouri al-Maliki “has gotten rid of most of the commanders who aren’t personally loyal to him and has staffed the forces with political hacks,” says retired Col. Peter Mansoor, who served as executive officer to retired Gen. David Petraeus during the Iraq surge.

This was part of Mr. Maliki’s effort to ensure that the military wouldn’t launch a coup against him. At the same time, however, it ensured “that the Army couldn’t fight effectively against a really capable military force, which ISIS clearly is,” says Mr. Mansoor, author of “Surge: My Journey With General David Petraeus and the Remaking of the Iraq War” and an associate professor of military history at Ohio State University in Columbus.

But ISIS will probably encounter a major roadblock on its proclaimed march to Baghdad, say current and former US military officials. “They are going to reach a natural culmination point: Their supply lines are going to get stretched, and they will enter an area with a much greater Shiite presence,” says Mr. Dubik, now a senior fellow at the Institute for the Study of War.

The predominantly Shiite nation of Iran is reportedly sending forces to help counter the Sunni ISIS fighters. ISIS is “not going to be able to take over Baghdad,” Mansoor says.

The resistance that ISIS will face could offer the US an opportunity, he adds. As the Iraqi government approaches the US for military aid, this gives US officials the leverage “to make Maliki get the politics right.”

Maliki’s sectarian agenda has created a splintered and unmotivated Iraqi Army, and to make it effective, it must be wrested from sectarian control. “Once that’s done, then we can go in with military support in the form of airstrikes, advisory teams, special forces, forward air controllers, and intelligence – and help them destroy [ISIS], which is an existential threat to Iraq,” Mansoor says.

The Pentagon has said that this is Iraq’s war to wage now. “Ultimately, this is an issue for Iraqi security forces to handle,” Rear Adm. John Kirby, Pentagon press secretary, said this week.

But if Iraqi security forces could handle the fight, “they would,” Dubik says. While Maliki’s sectarian domestic policies have created the problem and domestic politics must be part of the solution, the US should be willing to provide boots on the ground, he adds. “If ISIS is even partially successful” in holding the territories it has gained in western and northern Iraq, it could be on its way to creating “exactly the kind of sanctuary we’ve been fighting against since 9/11.”

A “small number” of US military planners could help to create a plan for a “properly executed defense and then transfer to a counteroffensive,” Dubik says, adding that the US should conduct airstrikes with US military aircraft and put tactical air combat controllers on the ground to make sure the strikes are accurate and don’t harm civilians.

There might even be political will among Americans for this sort of move if politicians make their case, argues Mansoor, who would like to see a “substantial” force of 10,000 to 12,000 US troops return to Iraq.

“We were the glue holding Iraq together, and unfortunately the glue hadn’t dried before we left,” he says. “I think the American people will understand.”

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Mr. Obama reiterated Friday that sending US combat troops to Iraq is not part of the calculus, but that he will be exploring “a range of other options” in the days to come.

Ultimately, however, Iraq must heed the wake-up call that is the ISIS military advance and “solve their problems,” Obama said. “Obviously our troops and the American people and the American taxpayers made huge investments and sacrifices in order to give Iraqis the opportunity to chart a better course, a better destiny,” he added. “They’re going to have to seize it.”