The Defence Department has enough forces in the Persian Gulf to conduct airstrikes that would probably prevent Sunni insurgents from marching into Baghdad, but other missions would be far more complex and risk drawing the United States back into an Iraqi civil war, according to retired military commanders.
The Pentagon announced that the USS Mesa Verde, an amphibious transport dock ship, had arrived in the gulf on Monday to join an aircraft carrier, a destroyer and a guided-missile cruiser. Together, the warships carry a large number of fighter jets and search-and-rescue aircraft, along with Tomahawk cruise missiles and other ordnance, that would give President Barack Obama an assortment of tactical options should he decide to take military action in Iraq.
"Militarily, we can do just about anything we want," said David Deptula, a retired Air Force lieutenant general who helped lead previous air campaigns over Iraq and Afghanistan. "The question is, to what end?"
The Pentagon's ability to deploy drones to conduct surveillance and carry out airstrikes — a move endorsed by many in Congress — may be limited. The US military has Predator and Reaper drones at several bases in the region but would have to get permission from reluctant host countries to use them in Iraq.
Targeting fighters from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) would not be difficult if they continue to advance toward Baghdad along highways and other visible routes, the former military commanders said. But airstrikes would become far more complicated if the insurgents stay within the cities they have taken control of in recent weeks, raising the likelihood of civilian casualties.
Obama on Friday ruled out sending ground forces "back into combat in Iraq". That decision means there may not be large numbers of US personnel in the country to help gather intelligence and identify targets for airstrikes. Unlike conventional military forces, the ISIL fighters don't operate from fixed bases and can easily blend in with noncombatants.
On Monday, US officials said the White House was considering sending up to 100 Special Operations troops to Iraq in a training and advisory role. Although the White House said those forces would not be directly involved in combat, it was unclear whether they could be used to select targets or call in airstrikes.
Without forces on the ground to verify that US airstrikes had hit legitimate military targets, the United States would become more vulnerable to enemy propaganda about mass civilian casualties, said Gary Roughead, a retired four-star admiral and chief of naval operations from 2007 to 2011.
"The other side gets to generate the narrative, whether it's fact or fiction," he said. "Not having the ability to coordinate from the ground makes it very hard."
For targets in populated areas, the United States would have to rely to a large extent on intelligence provided by the Iraqi military. Although the US military could also turn to satellite imagery and airborne surveillance, having troops on the ground to coordinate with Iraqi forces would greatly lessen the odds of a mistake, said James Poss, a retired Air Force major general who helped oversee the bombing of Afghanistan in 2001.
"It's the president's decision, and it's a pretty gut-wrenching decision to put a single soldier, airman or sailor on the ground," Poss said. "But I think to have a much higher confidence in the targeting and the source of intelligence, a presence on the ground may be necessary."
US military officials said the naval presence in the gulf puts plenty of firepower at Obama's disposal. The Defence Department also has several large air and naval installations in the region that played key roles during the last war in Iraq. But it is uncertain whether the countries that host those bases would allow them to be used for potential military action in Iraq this time around.
The Pentagon, for instance, has major air bases in Qatar, Kuwait and the United Arab Emirates. The rulers of those countries are Sunni Muslims and are generally leery of Iraqi Prime Minister Nouri al-Maliki, a Shiite whom the White House has criticised for exacerbating sectarian tensions.
Rear Admiral John Kirby, the Pentagon press secretary, declined to comment when asked if the US military was seeking permission from Persian Gulf countries to use their bases in the event that Obama decides to act in Iraq. He said that the basing agreements "vary from country to country, and I'm not going to speculate on that right now."
An inability to use the gulf bases would make it difficult for the Pentagon to deploy armed drones over Iraq. Predator and Reaper drones cannot take off from ships and lack the range to be effective from US bases in Afghanistan.
The US military has fighter aircraft and unarmed drones in Turkey, including a handful of Predators that have conducted surveillance flights over northern Iraq since 2011.
The Turkish government denied permission to use its bases for the 2003 invasion of Iraq, a lingering source of resentment in the Pentagon. More recently, relations have cooled between Washington and Ankara as Prime Minister Recep Tayyip Erdogan has cracked down on political opponents.
But Turkey may have stronger incentive to cooperate with the US military this time. Last week, ISIL insurgents overran the Turkish Consulate in the Iraqi city of Mosul and took scores of Turks hostage.
Samuel Brannen, a former senior defense official who served as the Pentagon's country director for Turkey, said cutting a deal with Ankara would be complicated but probably easier than winning permission from the gulf states to use the bases there.
"On the one hand, there is more willingness to cooperate on the part of Turkey," said Brannen, now a senior fellow at the Center for Strategic and International Studies in Washington. "On the other hand, doing anything with the Turks is very, very hard."
In Iraq, refugees from Mosul said they were fleeing their homes in anticipation of US airstrikes against ISIL fighters who have taken over the city.
"When we heard that the plane carriers had reached the gulf, we left," said Ahmad Ziad, a 30-year-old taxi driver who was leaving Mosul with his wife and two children, a cage carrying the family's pet bird perched on the front seat. "We aren't scared of ISIS; they are much better than the Iraqi army. We are scared of the airstrikes."
Some expressed frustration that the international community or the Iraqi government had not acted to strike the militants sooner.
"They should have struck [ISIL] when they were outside the towns and cities — now they are among the people," said Mohammed Abdullah, a local council member from the town of Hatra, southwest of Mosul. "How are they going to tell who is who?"