WASHINGTON – When it comes to Syria, the Obama administration is sure about one thing: President Bashar Assad's government must be punished after allegedly using deadly chemical weapons, possibly including sarin gas, to kill hundreds of Syrians.
The U.S. and allies accuse Assad of crossing a line that President Barack Obama said would have "enormous consequences." That's now expected to trigger a military strike, limited in time and scope, with the goal of downgrading and weakening Assad but not toppling him or destroying his forces.
The details of how and when the U.S. military and allied forces might attack are under debate but would be based on complex plans developed and repeatedly reworked over time by the Pentagon.
A look at what's known and what's unclear about how it might unfold.
The order for the strike would come from Obama, delivered to Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, and Defense Secretary Chuck Hagel. The operation probably would fall under the purview of U.S. Central Command, headed by Army Gen. Lloyd Austin. The more immediate commander probably would be Adm. Bruce Clingan, who heads U.S. naval forces in Europe.
U.S. commanders would communicate and coordinate with military officers from other nations involved in the fight, such as France.
WHO LAUNCHES WHAT
Five U.S. Navy destroyers — the USS Gravely, USS Mahan, USS Barry, the USS Stout and USS Ramage — are in the eastern Mediterranean Sea waiting for the order to launch. And the USS San Antonio, an amphibious assault ship has now joined them. The USS San Antonio, which is carrying helicopters and Marines, has no cruise missiles, so it is not expected to participate in the attack. Instead, the ship's long-planned transit across the Mediterranean was interrupted so that it could remain in the area to help if needed.
The destroyers are armed with dozens of Tomahawk cruise missiles, which have a range of about 1,000 nautical miles and are used for deep, precise targeting. Each one is about 20 feet long and less than two feet in diameter and carries a 1,000 pound warhead.
The missiles fly at low altitudes, and their range allows the ships to sit far off the coast, out of range of any potential response by the Syrian government. Some ships have cameras that can provide battle damage assessments.
The Navy also now has two aircraft carriers in the Arabian Sea that are loaded with fighter jets. The USS Truman arrived in the region to take the place of the USS Nimitz, which was supposed to head home. But the Navy ordered the Nimitz to stay for now.
U.S. officials described the decision as prudent planning and said it doesn't suggest the Nimitz would play a role in any possible strikes in Syria.
With Britain on the sidelines, France has said it is preparing for military action against Syria. French President Francois Hollande does not need parliamentary approval to launch a military operation that lasts less than four months.
French military officials confirmed the frigate Chevalier Paul, which specializes in anti-missile capabilities, as well as the hulking transport ship Dixmude, had set off Thursday from the Mediterranean port of Toulon as part of normal training and operational preparations — but denied any link to possible Syria operations.
France also has a dozen cruise missile-capable fighter aircraft at military bases in the United Arab Emirates and the Horn of Africa nation of Djibouti, as well as fighters that could launch from air bases in the French island of Corsica or western France.
HOW DO THE NATIONS COORDINATE?
Details are unknown about how the mission strikes are being allocated or if the U.S. and France have mapped out separate, agreed upon target lists. But the U.S. routinely conducts exercises with allies, particularly NATO countries such as Britain and France, in which they all practice exactly this type of joint attack mission.
Commanders have a wide variety of ways they can talk to each other, including through integrated communications systems honed over many years of NATO operations ranging from the Afghanistan war to the 2011 attack on Libya and the fighting in Algeria and Mali early this year.
The military officers can speak or email across classified, secure lines and even have systems that allow them to talk in real time in Internet chat rooms. The nations also often have military liaisons embedded with each other to help assist communications.
Because any operation is expected to be limited, there likely won't be more organized, formal war rooms.
WHAT ABOUT TROOPS, FIGHTER JETS AND BOMBERS?
Obama has ruled out putting troops on the ground in Syria, and because of Assad's extensive air defense systems, officials believe it is too risky, at least initially, to deploy fighter aircraft or even low-flying drones that could be shot down.
While less likely, the U.S. could deploy fighter jets or bombers as the operation continues, particularly if the Assad regime begins to take retaliatory actions and manned aircraft are needed in order to strike specific, critical targets.
Obama has rejected trying to impose a "no-fly" zone over the country. Military leaders have said that creating one would be risky and expensive.
WHAT MIGHT THEY TARGET?
U.S. officials say any operation must have clear goals that can guide decisions on what the military must strike.
Dempsey has told Congress that lethal force would be used "to strike targets that enable the regime to conduct military operations, proliferate advanced weapons and defend itself."
At a minimum, Western forces are expected to strike targets that symbolize Assad's military and political might: military and national police headquarters, including the Defense Ministry; the Syrian military's general staff; and the four-brigade Republican Guard that is in charge of protecting Damascus, Assad's seat of power. Assad's ruling Baath Party headquarters could be targeted, too.
U.S. officials also are considering attacking military command centers and vital forces, communications hubs and weapons caches, including ballistic missile batteries.
Air defense systems, including Syrian aircraft, interception missiles, radar and other equipment, also could be targets. The majority of those systems — as many as 500 defense positions and 400 operational aircraft — have been positioned along Lebanon's border, in the Syrian-controlled part of the Golan Heights, along the Syrian Mediterranean coast and in and around Damascus.
Helicopter and fixed wing aircraft air bases across the country, including the Mezzeh air base in Damascus, and Nairab, a major military air base in Aleppo, could be targets.
Because any strike would be considered payback for Assad's alleged use of chemical weapons, Western forces could zero in on the headquarters of the Syrian Army's 4th Division, 155th Brigade. That unit is believed to have been responsible for the Aug. 21 attack that U.S. officials say involved chemical weapons. The brigade is headed by Maher Assad, Bashar Assad's younger brother.
The brigade has a missile base across a large terrain in a mountain range west of Damascus, including underground bunkers and tunnels. It is believed to be surrounded by army bases as well as weapons and ammunition storage sites.
Systems for moving Assad's chemical weapons stockpile could be top targets as well. But the stockpile itself probably would not be hit because of risk of accidental release of deadly nerve agents that include mustard gas, tabun, sarin and VX.
WHAT PROBABLY WOULD BE AVOIDED?
It's doubtful the U.S. would directly target Assad. U.S. policy prohibits assassinating foreign leaders unless they have attacked America first.
It's also unclear if Assad's military intelligence headquarters, a symbolic target, might be attacked; it's believed to hold hundreds of prisoners.
WHEN MIGHT A STRIKE COME AND HOW LONG MIGHT AN ATTACK LAST?
The most common answer to this question in recent days has been "soon." But a number of factors could affect the timing.
U.N. inspectors wrapped up their investigation into the suspected chemical attack and left Syria on Saturday. And officials say they are still talking to allies.
There has been a so-far unsuccessful effort to seek U.N. Security Council approval for a strike, but there is also significant pressure on the administration to act quickly and decisively.
Any military operation would probably unfold at night or in the predawn hours in Syria, with an initial assault possibly lasting several hours and involving dozens of missile strikes from several warships.
What could follow is a period in which the U.S. would use satellites and other intelligence capabilities to assess the damage.
Such an assessment could be followed by an additional round or two of missile strikes, if ordered by the president. Officials believe the strikes could be limited to a single operation, but if extended would likely last no more than a few days.
Other U.S. military assets in the region, including an Air Force air wing of F-16 fighter jets located in Aviano, Italy, are available but might not be used, at least right away.
WHAT ABOUT THE SYRIAN MILITARY?
The Assad regime is believed to have about 400 operational aircraft and one of the most robust air defense networks in the region. There are multiple surface-to-air missiles providing overlapping coverage of key areas in combination with thousands of anti-aircraft guns capable of engaging attacking aircraft at lower levels.
Syria also has a mobile, land-based coastal defense system, including Yakhont anti-ship missiles capable of sinking large warships, including aircraft carriers.
Two years ago, the standing army was estimated to be about 250,000, but if reserves are included it could number closer to 700,000. The last two years of civil war, however, have taken a toll on the military, due to defections and the ongoing warfare.
The biggest concern, however, is that any U.S. attack could prompt retaliation by Assad, including the possible use of chemical weapons against Syrian citizens or even attacks on nearby nations.
Associated Press writers Zeina Karam in Beirut, Shawn Pogatchnik in Dublin and Sylvie Corbet and Greg Keller in Paris contributed to this report.