With the fall of Mosul on Tuesday, Iraq’s al Qaeda offshoot has not only seized the country’s second-largest city, it appears it also has come into possession of the heavy weapons and vehicles the U.S. military had provided Iraq’s military to fight them.

That’s terrible news for America’s few allies left in Iraq as the Islamic State of Iraq and the Sham (ISIS) morph from terrorist menace to a military force capable of over-running an army the U.S. military trained for nearly a decade. It also calls into question the American government’s decision to withdraw the last of its forces from Iraq in 2011. Three years later that withdrawal now appears premature.

ISIS themselves appear to be pleased with the booty of the Mosul campaign. On Twitter the group posted a photo of a beaming Umar al-Shishani, a senior ISIS commander, examining a captured American-made Humvee, which had already been driven from the Iraqi city to the eastern Syrian town of Deir ez-Zor. “Umar al-Shishani inspects spoils of war…Looks quite pleased,” the tweet said.

General Najim al-Jabouri, a former mayor of Tel Afar, which is a little more than 31 miles from Mosul, told The Daily Beast the bases seized by ISIS this week would provide the group with even more heavy weapons than they currently control. “The Iraqi army left helicopters, humvees, cargo planes and other heavy machine guns, along with body armor and uniforms,” the general, who is now a scholar at the National Defense University, said. He said he was able to learn about the equipment from soldiers and other politicians in and around Mosul with whom he keeps in touch.

General Najim is not alone in this assessment. Jack Keane, a retired four-star Army general who was a key adviser to General David Petraeus during the counter-insurgency campaign in Iraq in 2007 and 2008 known as the surge, said ISIS has now established itself as a formidable military force.

“This organization has grown into a military organization that is no longer conducting terrorist activities exclusively but is conducting conventional military operations,” he said. “They are attacking Iraqi military positions with company- and battalion-size formations. And in the face of that the Iraqi security forces have not been able to stand up to it.”

There were signs this was coming. Sheik Jassim Muhammad Suwaydawi, one of the remaining pro-American tribal leaders in western Iraq, told The Daily Beast on Tuesday that his forces have fought ISIS companies that drove tanks in western Iraq. “ISIS is using heavy weapons, in some cases even tanks because of the weakness of the Iraqi Army.”

It was not supposed to be this way. Back in 2008, U.S. counter-insurgency commanders touted the Iraqi city of Mosul, 200 miles northwest of Baghdad, as a model success in the American “troop surge” that helped contain the Sunni-Shia civil war. But today Mosul became the de facto capital of a jihadist newcomer that is not only crumbling the authority of the Iraqi government but also overshadowing al Qaeda.

The siege of Mosul by ISIS, who overran the northern city on Tuesday after Iraqi forces fled posts and government buildings, also amounts to a severe blow to the government of Nouri al-Maliki. The Iraqi government has floundered in its efforts to stop the country from sliding back into a vicious spiral of bloodshed that claimed more than 8,800 lives last year alone.

Following the loss of his country’s second-largest city, the hard-pressed Iraqi prime minister is now turning to the U.S. for more assistance, reportedly asking the Obama administration for missiles and artillery—although he has not asked for a return of U.S. troops, a request unlikely to be viewed favorably either in Washington, D.C., or by Maliki’s Shia Muslim supporters.

Sheik Jassim told The Daily Beast. “The situation on the ground has been deteriorating for the last seven months, but in the last couple of days it has significantly gotten worse.” He added, “We in Anbar want U.S. air and military support because what happened in Mosul could happen in Anbar and we want to prevent it.”

The resurrection of ISIS in Iraq began a year ago. Last summer ISIS staged a successful jailbreak at the infamous Abu Ghraib prison, freeing many of its leaders. After the jailbreak ISIS not only expanded into neighboring Syria while breaking away from al Qaeda’s central command in Pakistan, the group also took Fallujah, the site of one of the Iraq war’s most infamous battles in 2004.

At this point Iraqi leaders in Baghdad are considering asking the Kurdish militia known as the Peshmerga to intervene in Mosul. A senior official with the Kurdistan Regional Government told The Daily Beast that lower-level Iraqi officials have floated the idea today of Kurdish forces stepping into Mosul, a Sunni Arab majority city that is close to Kurdish territory, to try to restore order. This official said: “We have significant interests and assets in the region,” although he noted that ISIS has not attacked Kurdish targets in western Iraq.

That isn’t the case in Syria, where Syrian Kurds have been locked in a ferocious months-long struggle with ISIS and have been unable to maintain the advances they were managing in the winter.

“The jihadists have mounted several attacks in recent weeks,” Kurdish activist Kovan Direj told The Daily Beast in a phone interview.

The capture of Mosul won’t make life any easier for the Syrian Kurds. It is a further major step in the establishing of ISIS, which is made up at its core of battle-hardened Iraqis but has attracted fighters from across the Middle East, Central Asia and Europe, as the most successful jihadi group in history in terms of strategic territory controlled or land battles won.

ISIS’s rise since its formation in 2010 by Abu Bakr al-Baghdadi has taken many analysts by surprise: Premature obituaries were written when the jihadi group, which emerged from the Islamist insurgency in Iraq, was disowned this year by al Qaeda’s top leaders, who were furious with its brazen refusal to obey orders to withdraw to Iraq, allowing the compliant Jabhat al-Nusra the unchallenged al Qaeda franchise in Syria.

The Iraqi government is not the only one seemingly unable to halt the expansion of ISIS, which has grabbed a swath of cross-border land stretching from Iraq’s Mosul up through Iraq’s Anbar province and all the way west to the Syrian town of Al Bab on the outskirts of Aleppo.

Since being disavowed in February, ISIS has fiercely competed with al Qaeda to secure the allegiance of affiliates and jihadi groups across the Middle East. And for months jihadi religious scholars have been waging theological debate and lining up behind core al Qaeda’s leader Ayman al-Zawahiri or al-Baghdadi in the struggle for jihadi supremacy.

Success breeds success, say analysts. The dramatic seizing of Mosul will only add to ISIS’s luster, helping it to recruit more fighters as it seeks to carve out a “caliphate” across western Iraq and eastern Syria—much as 9/11 was a recruitment driver for al Qaeda. Jihadist social media sites were jubilant Tuesday. “Jihadis are ecstatic with ISIS’s achievement,” say researchers at the Middle East Media Research Institute, a Washington-D.C.-based nonprofit.

Mosul’s capture is being presented by ISIS as a validation of al-Baghdadi’s jihad strategy, one that focuses on controlling territory and proto-state building. Al Qaeda in its videos and websites focuses on global jihad; ISIS in its propaganda celebrates towns captured and land controlled, notes Pieter Nanninga, a Mideast scholar at the University of Groningen in The Netherlands.

There are also funding and logistical imperatives for ISIS to pursue the proto-state strategy. The group has been able to fund itself by running extortion rackets in the towns it controls in Iraq and Syria and from oil smuggling—essential revenue streams with al Qaeda dominating funding from jihadist-sympathizers in the Gulf.

With its overrunning of Mosul ISIS has taken a significant step in its carving out of a caliphate that is reshaping both Iraq and Syria.

Nadette de Visser contributed to this article.