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 the 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. A coalition likely joined by allies such as Britain and France would likely set up a joint task force to coordinate targeting and missions.
WHO LAUNCHES WHAT
Four U.S. Navy destroyers -- the USS Gravely, USS Mahan, USS Barry and USS Ramage -- are in the eastern Mediterranean Sea waiting for the order to launch.
They 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 region, loaded with fighter jets. The USS Truman arrived in the Arabian Sea to take the place of the USS Nimitz, which was supposed to head home. But the Navy ordered the Nimitz, which is in the Indian Ocean, to stay for now.
U.S. officials described the decision as prudent planning, and said it doesn't suggest the carrier would play a role in any possible strikes in Syria.
Britain's Royal Navy has deployed at least one Trafalgar-class attack submarine to the Mediterranean, though it declines to specify where. Each of the subs typically carries around a dozen Tomahawk missiles. That supply, if exhausted, could be quickly restocked by a Royal Navy nine-ship task force that deployed this month to the eastern Mediterranean.
The British Royal Air Force base Akrotiri, 175 miles west of the Syrian coast, could be used by cruise missile-capable U.S., British and French aircraft.
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,