WASHINGTON — A coming Iraqi offensive to drive the Islamic State out of Iraq's second-largest city renews a debate on whether U.S. forces should play a larger role in the operation despite the risk of drawing them back into a war.
The White House has pledged to keep American forces out of combat, but Iraq would suffer a major setback in Mosul if its troops falter because U.S. advisers are restricted in what they can do, security analysts say.
Failure to retake Mosul from the militants would "reverse all the gains we made since August," when the United States launched airstrikes to stem the Islamic State's advances in Iraq, said James Jeffrey, a former U.S. ambassador to Iraq.
"A lot is riding on this operation," said David Barno, a retired lieutenant general.
There are about 3,000 U.S. troops in Iraq, including hundreds of trainers and advisers. They are limited largely to protected bases, not any battlefield, because of White House concern that an expanded military role could lead to "mission creep," the slow expansion of involvement in another war four years after U.S. combat troops withdrew from Iraq.
President Obama's proposed legislation authorizing military force against the Islamic State would prohibit "enduring offensive ground combat operations."
Since the United States began bombing Islamic State militants in Iraq last summer, it has conducted the air campaign without the benefit of American teams on the battlefield, which are typically used to control airstrikes in support of ground forces.
The Pentagon says the restrictions have not hampered the effectiveness of the bombing campaign, which helped drive militants from Kobani, a Syrian city on the Turkish border.
Liberating Mosul will be a different challenge. Civilians largely evacuated Kobani, but hundreds of thousands of residents remain in Mosul. That makes precision airstrikes critical, since the militants will probably attempt to hide among civilians.
Republican lawmakers, such as Sen. John McCain of Arizona, call on the White House to allow Special Forces teams to accompany Iraqi troops, so they can help call in airstrikes and aid in other functions. The GOP critics have complained that Obama has placed too many restrictions on the U.S. military in Iraq.
Military commanders have not weighed in on lifting the restrictions. They may wait to see how the Mosul offensive unfolds before making a recommendation, said a senior Pentagon official, who asked not to be named because he isn't authorized to discuss the plans.
Fred Kagan, a military analyst at the American Enterprise Institute, said the ban on letting American advisers accompany Iraqi troops on the battlefield is a self-imposed "red line." He said placing air controllers with Iraqi forces would make airstrikes more effective, though the mission could be accomplished without them.
U.S. forces use drones and Iraqi ground forces to identify targets, which are cleared for bombing by American officers in combat operation centers miles from the battlefield. That has been effective against militants in military vehicles or bunkers.
"Thus far, we have been able to accomplish what we need to do with the current system of providing air support," said Maj. Curtis Kellogg, a spokesman for U.S. Central Command.
Islamic State forces may fight to hold Mosul using civilians as human shields, rather than attempt to flee. The militants have declared the region part of their "caliphate" and set up shadow governments.
U.S. Central Command expects a ground force of 20,000 to 25,000 Iraqi and Kurdish troops to free Mosul. To arm them, the Pentagon is rushing arms and ammunition to Iraq, including a $17.9 million shipment this week.
Though the attacking force will vastly outnumber the 1,000-2,000 Islamic State fighters in Mosul, the militants will probably rig buildings to explode as part of their effort to defend the city, said Kagan, who recently visited Iraq.
Barno said the Islamic State "may actually find this a place where they can fight effectively."