 -- President Obama said today that Kurdish and Iraqi forces have retaken the Mosul Dam in Iraq, a decaying, dangerous structure that, with or without brutal extremists in charge, threatens millions of people downstream.

Earlier today Gen. Karim Fatah, commander of a Kurdish peshmerga battalion near the dam, told ABC News Kurdish troops had retaken key parts of the dam from the extremist Islamic State of Iraq and Syria (ISIS) but the battle was ongoing. The Iraqi military followed with a similar statement. The U.S. military said it conducted more than a dozen airstrikes today in support of the Kurdish and Iraqi troops.

Just over an hour before the president spoke, a spokesperson for the U.S. State Department said only that there had been “progress around the dam” and that the operation was “ongoing.”

President Obama is the highest level official to say without equivocation that the U.S.-backed forces had retaken the dam – calling it a “major step forward.”

ISIS managed to take control of the dam last week, an eventuality about which a spokesperson for the U.S. State Department had previously said the U.S. government was “extremely concerned.”

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Sunday President Obama sent a letter to Congress notifying lawmakers that he had authorized airstrikes against ISIS targets at the dam “in the national security and foreign policy interests of the United States.”

“The failure of the Mosul Dam could threaten the lives of large numbers of civilians, endanger U.S. personnel and facilities, including the U.S. Embassy in Baghdad, and prevent the Iraqi government from providing critical services to the Iraqi populace,” the letter said.

The stark language actually may have downplayed the danger posed by the dam, according to prior U.S. estimates of the damage that could be caused should the dam be breached – or even if it is simply left alone to degrade on its own without the constant repair work that has been critical to keeping the dam right side up for the past 30 years.

The Mosul Dam was built in the mid-1980s on what reports indicate was a terrible spot to build a sprawling dam.

“Mosul Dam, the largest dam in Iraq, was constructed on a foundation of soluble soils that are continuously dissolving, resulting in the formation of cavities and voids underground that place the dam at risk for failure,” said an urgent letter sent from David Petraeus, then commanding general of the U.S. Army, and Ryan Crocker, then U.S. Ambassador to Iraq, to Iraqi Prime Minister Nouri al-Maliki in 2007.

The dam requires “extraordinary engineering measures” -- namely constant grouting operations -- to fill in the holes and “maintain the structural integrity and operating capability of the dam,” according to a U.S. Army Corps of Engineers (USACE) report from the same year.

For 30 years –- and through several periods of violent conflict -- the Iraqi government has managed to keep the dam upright by continuously pumping in literally tons of grout like an industrial version of the little Dutch boy, as a geotechnical expert who worked on the dam put it.

But the U.S. says any failure of the dam could be “catastrophic.”

“[T]he most severe impact of a dam failure would be [for] the City of Mosul, located 50 kilometers [31 miles] downstream of the dam,” Petraeus’ and Crocker’s 2007 letter said. “Assuming a worse [sic] case scenario, an instantaneous failure of Mosul Dam filled to its maximum operating level could result in a flood wave over 20 meters [65 feet] deep at the city of Mosul, which would result in a significant loss of life and property.”

Mosul is estimated to be home to more than 1.5 million people. Flood waters, albeit at a lower level, could reach all the way to Baghdad, more than 200 miles further down the Tigris.

A 2011 report written by a USACE official and published in Water Power magazine estimated failure “could lead to as many as 500,000 civilian deaths.”

Recently a U.S. official confirmed that the dire 2007 estimate still stands. After Mosul, flood waters would travel for eight to 10 days before reaching Baghdad, where the U.S. Embassy there could see one to four meters of water, the official said.

The U.S. State Department said earlier this month that control of the dam was one of ISIS’s goals in Iraq. Late last week the extremist group got its wish, took control of the dam and immediately inherited the urgent grouting problems.

Friday an Iraqi government official said that the lead dam engineer and his team were still on site and operating the dam at ISIS’s behest. Supplies to continue grouting operations were available and the water level was also being kept lower than normal to reduce the risk of a breach, the official said then.

ISIS may not necessarily want the dam to fail, considering the extremist group controls portions of the land that would be flooded. The dam is also a “key source” of power and water for the surrounding area, making it a vital piece of infrastructure either way for whoever is in control, another State Department spokesperson told ABC News last week.

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