WASHINGTON • Recent gains against the Islamic State in Iraq and Syria (ISIS) in eastern Syria have helped sever critical supply lines to Iraq and set the stage for what will be the biggest fight yet against the Sunni militancy, the battle to retake Mosul, the Pentagon says.
General Joseph Dunford, chairman of the Joint Chiefs of Staff, on Monday said US-backed forces had begun laying the groundwork for the fight by moving to isolate Mosul from ISIS' de facto headquarters in Raqqa.
Kurdish and Arab forces retook the town of Shaddadi in eastern Syria last week, cutting off what US Defence Secretary Ash Carter called the last major artery between Raqqa and Mosul.
But military officials cautioned that the fight for Mosul could last months, requiring Iraqi forces unproven in urban warfare to advance street by street through the explosives-laden terrain of Iraq's second-largest city with a population of more than one million.
The Pentagon has also begun cyber attacks on ISIS communications between Raqqa and Mosul, as well as attacks meant to disrupt the group's ability to use social media to recruit fighters, officials said.
When the coalition cleared Fallujah, it took forever, and Mosul is larger than Fallujah.
MR PATRICK MARTIN, an Iraq expert at the Institute for the Study of War.
Retaking Mosul would be a "massive hit" to ISIS, said Mr Patrick Martin, an Iraq expert at the Institute for the Study of War. Such a loss would bolster claims by the US-led coalition that the Sunni militancy is on the run in Iraq.
It could also sharply demoralise ISIS fighters, raising questions about whether the group could still credibly call itself a caliphate.
The Pentagon has declined to predict when Iraqi troops will try to enter Mosul, though Gen Dunford said "it is not something that will happen in the deep, deep future".
Iraq Prime Minister Haider al-Abadi said two weeks ago that Iraqi forces would start a full military operation to retake the city as early as this month, and an American military official said over the weekend that the Pentagon believed Iraqi troops were ready to launch a credible assault.
Still, officials acknowledge that the battle will be an uphill slog.
"Do I think it is going to be easy? No," Major-General Richard Clarke, the commander of US land forces in Iraq, said last week.
"It is going to be tough."
The long fight by Iraqi security forces to take back Ramadi from ISIS, which concluded in December, offers a preview of the battle to come over Mosul.
Advancing inch by inch, Iraqi forces, backed by US air strikes, took more than five months to gain control of the city centre of Ramadi, the capital of Anbar province.
As difficult as that battle was, the fight for Mosul will be much harder, military officials say. The city is five times as large as Ramadi. And while the Iraqi military used two US-trained brigades in the Ramadi fight - the 73rd and the 76th, which Gen Clarke said were believed to be the best in the Iraqi army - those forces number some 8,000, far short of the 30,000 troops Pentagon officials say are needed.
Military officials also say it is hard to imagine how the fight for Mosul can be waged without close US air support, which would probably require attack helicopters, something Dr Abadi, for political reasons, has yet to agree to.
The effort is likely to include Kurdish peshmerga fighters, Pentagon officials say.
The US military has trained some 16,000 Kurdish fighters, but Dr Abadi's government is unlikely to want the Kurdish fighters to assume the lead role in the coming fight, a role that Iraq experts say is likely to be filled by the Shi'ite-dominated Iraqi security forces.
In 2004, it took more than 13,000 highly trained troops, primarily Americans, almost two months to retake and clear Fallujah of about 3,000 insurgents in the fiercest fight of the Iraq war. Ninety-five US service members died, and more than 560 were wounded.
The battle for Mosul, many military experts say, could be much worse. Pentagon officials say they are unsure how many ISIS militants are in the city, but they have been there for almost two years.
"When the coalition cleared Fallujah, it took forever, and Mosul is larger than Fallujah," Mr Martin, the Iraq expert, said.
"And the people who will be doing the clearing are not US troops."
Meanwhile, ISIS attacks on the ground are not abating. Four ISIS suicide bombers on Monday infiltrated an army headquarters in the Haditha area of Anbar province, west of Baghdad, killing an Iraqi general and five other soldiers, army and police officers said yesterday.
NEW YORK TIMES, AGENCE FRANCE-PRESSE