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 IRBIL, Iraq —

Six months ago, Sunni Arab extremists faced a daunting firepower imbalance in their uprising against the U.S.-equipped Iraqi army west of Baghdad.

But once their campaign for the city of Fallujah was launched in January, their lethal capabilities were bolstered from the stockpiles of the Iraqi armed forces. Many soldiers fled, throwing down their weapons, which were picked up by the insurgents.

Police stations and security posts overrun by Sunni extremists yielded more martial booty to be turned against the forces of Prime Minister Nouri Maliki’s Shiite Muslim-led government.

“Praise Allah, we soon had enough weapons to fight for one or two years,” said Ahmad Dabaash, spokesman for the Islamic Army, a Sunni rebel faction, who spoke in a hotel lobby here in Iraq’s northern Kurdistan region. “And now? Don’t even ask!”

By “now,” he was referring to the ground assault by the Islamic State of Iraq and Syria (ISIS), the al-Qaida breakaway group that in the past two weeks has seized large parts of northern and central Iraq, including Mosul, Iraq’s second-most populous city.

Fighting alongside ISIS formations are other Iraqi Sunni factions such as the Islamic Army, which rose against the U.S. occupation a decade ago.

As the Iraqi government mobilizes to halt the insurgents’ advance toward Baghdad, there is no full accounting of the stocks of plundered arms, ordnance and gear. But experts agree the haul is huge — with implications for the merging wars in both Iraq and neighboring Syria.

Rival Syrian rebel factions already report seeing U.S.-built, ISIS-commandeered Humvees almost as far east as the vicinity of Aleppo, 250 miles from Iraq. The influx of arms and fighters from Iraq could shift the balance of power among fractious rebel groups fighting for supremacy in Syria.

ISIS, which also reportedly snatched the equivalent of close to $500 million in cash from a Mosul bank, has been catapulted to the position of the world’s wealthiest and best-equipped extremist group, analysts say. Its riches easily eclipse those of al-Qaida under Osama bin Laden, despite his personal fortune

The group, which has attracted thousands of fighters from the Arab world, Europe and elsewhere, also controls a broad swath of contiguous territory in the heart of the Middle East.

“ISIS are well-trained, very capable and have advanced weapons systems that they know how to use,” said Michael Stephens, researcher at the Royal United Services Institute for Defense and Security Studies.

In the current ISIS-led thrust, the scenario played out earlier by Sunni insurgents in western Iraq has been replicated on a monumental scale.

Government forces retreated en masse from the onslaught, leaving behind a military hardware bonanza, including the U.S.-made armored Humvees as well as trucks, rockets, artillery pieces, rifles, ammunition and even a helicopter. Some of the seized materiel was old or otherwise nonfunctioning; but a lot was promptly put to use on the battlefield.

Pictures of grinning Islamist warriors cruising in U.S. Humvees bedecked with white-on-black extremist flags flooded the Internet and became the signature image of the ISIS campaign.

Though ISIS initially encountered little opposition from the Iraqi army in central and western Iraq, it has not directly challenged Kurdish troops known as the peshmerga who control a more than 600-mile front in northern Iraq.

Stretching from the Syrian to Iranian borders, this territory is protected by the semiautonomous Kurdistan Regional Government. Iraqi soldiers who once patrolled much of the line retreated and are now found only along about a 35-mile stretch close to Iran, according to Kurdish security officials.

ISIS “took the weapons stores of the 2nd and 3rd (Iraqi army) divisions in Mosul, the 4th division in Salah al Din, the 12th division in the areas near Kirkuk, and another division in Diyala,” said Jabbar Yawar, secretary-general of the Ministry of Peshmerga Affairs. On a map, he indicated an arc denoting various provinces and cities strung across northern and central Iraq.

“We’re talking about armaments for 200,000 soldiers, all from the Americans,” Yawar said.

With such an immense quantity of captured weaponry, Yawar said, ISIS and its confederates are now capable of laying down “a colossal intensity of bullets” against their foes.

The plundered weapons and likely flood of new recruits might shift the initiative among rebel groups in neighboring Syria. ISIS emerged from the turmoil of the Syrian conflict but later suffered setbacks in internecine rebel combat.

This year ISIS faced an assault from rival insurgent factions that cut its presence to a few strongholds in northern and eastern Syria, including the city of Raqqah. Various Syrian rebel groups, including the Islamic Front and the Nusra Front, the latter the al-Qaida franchise in Syria, are avowed enemies of ISIS, which broke away last year in a bitter dispute.

But the newly galvanized ISIS recently made substantial gains along the desert borderlands of the Iraqi-Syrian frontier. It seized the border town of Qaim and tore down border fences and bulldozed berms and ditches in a dramatic gesture meant to illustrate its goal of creating a unified Islamist caliphate. ISIS forces also advanced near the eastern Syrian city of Dair Alzour, capital of the oil-rich province of the same name.

The lightning assault and attendant publicity may be winning new allies, even among the Nusra Front.

Several days ago, a group of Al Nusra rebels in the Syrian town of Bokamal, along the Euphrates River on the border with Iraq, pledged allegiance to ISIS, according to various accounts.

ISIS-captured Humvees helped alter the balance of power on the border battlefield, said a Nusra fighter reached via Skype.

More ISIS fighters and weapons are expected to pour into Syria from Iraq, said Col. Abdulrazzaq Abu Bilal, commander with Liwa Tawheed, one of the Syrian rebel groups aligned against ISIS.

ISIS has been massing forces north of Aleppo and clashing with rival rebel groups for a week just 12 miles from the main highway linking Aleppo with Turkey, Bilal said via Skype from Syria.

“After the Iraqi borders opened and ISIS seized control of the Dair Alzour suburbs, this gave them the motivation to advance toward Aleppo,” said the rebel colonel, a defector from the Syrian air force.

The group’s successes have prompted President Obama to seek $500 million from Congress to train “appropriately vetted elements of the moderate Syrian opposition.”

As ISIS continues to storm through Iraq, the rebel colonel said, its leaders seem determined to repeat the same offensive trajectory in Syria — and regain areas ceded to rivals in northern Syria. “They are seeking to control the Turkish border in its entirety,” he said, and “to cut off the supply routes and retake all the areas they lost.”