As U.N. officials negotiate terms of their probe into murky reports of chemical weapons use in Syria, one thing remains clear: An attempt by the United States to seize Syria’s deadly stockpile would be a difficult and dangerous undertaking.

Any successful move to secure the stockpile would likely involve thousands of U.S. troops and a significant commitment of air power to handle numerous tactical challenges. In addition to the risk of potential U.S. casualties at some well-defended sites, the cost for such an operation could easily run into the billions of dollars. There is also the inherent danger of setting off the chemical weapons during an operation—or prompting a desperate Assad regime to actually use its chemical weapons as a trump card.

“The more they look at it, the more difficult the mission appears,” said Gary Samore, who worked on the Syrian chemical weapons issue as White House Coordinator for Arms Control and the Weapons of Mass Destruction. “It would be a huge, complicated undertaking in which all kinds of things could go wrong,” Samore told Defense One.

“I don’t know if it would be a campaign, but it would be a very sophisticated, multi-service operation,” said former Deputy Commander of the U.S. European Command Charles Wald. The mission would require knocking out Syrian air defenses and heavy use of special operations forces.

The military has the capability to pull off such an operation. “I can guarantee you that it would not be textbook perfect, but it can be done,” Wald said, but ”can you accept the fact that the results are not so pretty?”

Joint Chiefs of Staff Chairman Gen. Martin Dempsey laid out a discouraging scenario when describing the challenges for seizing Syrian chemical weapons in a recent unclassified memo to Congress. “Thousands of special operations forces and other ground forces would be needed to assault and secure critical sites,” Dempsey wrote. This would entail the “added risks of boots on the ground” along with air cover. “Costs could also average well over one billion dollars per month.”

But perhaps Dempsey’s biggest warning was the possibility of less than 100 percent success in controlling weapons of mass destruction. “The impact would be the control of some, but not all chemical weapons,” the chairman predicted.

And those are just some of the concerns. There’s a whole host of challenges involved with seizing the weapons:

It’s hard to pinpoint the weapons sites. The Syrians reportedly have moved chemical weapons materials during the civil war.“You need to have very precise information,” Samore explained. Syria is believed to possess deadly sarin and VX nerve agents, as well as mustard gas. “Keep in mind that the Syrian arsenal is vast, with several different types of chemical agents, stored in several ways” including weaponized munitions.

“If you add in everything, which is storage, production, filling stations, probably you’re talking about a couple of dozen locations of different types, guarded differently,” Samore said. “To carry out something like a simultaneous attack would be difficult.”

Bombing the stockpiles could worsen the situation. Air power alone appears to be off the table, according to the plans described by Dempsey and analysts. “I know that they have looked at chemical defeat munitions that would incinerate weapons stockpiles,” GlobalSecurity.org director John Pike said, adding, “I have not heard a great deal of enthusiasm on that topic.” The oft-expressed fear is that bombs could also wind up dispersing chemical weapons or agents in the air. “It’s a nasty business,” noted one congressional expert. “You could end up spreading the chemicals.” Civilian populations could be affected. “You’re not going to do it by blowing it up because you can’t assure the consequences,” Wald said.

We have to rely on our allies for operational security. The U.S. might conduct a joint operation with NATO allies. Also, Jordan, Israel, Saudi Arabia and others in the Mideast region cooperate in everything from intelligence sharing to launching U.S. planes and forces from bases in the region. “There is a huge operational security issue” with all the planning involved, Wald said.

Setting up a no-fly zone won’t be easy, either. Unlike a single clandestine raid, dominance over Syria airspace would be critical in allowing troops to arrive safely. “At a minimum, this option would call for a no-fly zone as well as air and missile strikes involving hundreds of aircraft, ships, submarines, and other enablers,” Dempsey said in his memo. Special Operations forces would likely play a major role, since their experience and training is designed for these kinds of missions. “If I were planning it, I would have the special operations forces take the primary lead of planning it and executing it,” says Wald, “and everybody else would be in support.” Wald said Marine Corps prowess in dealing with chemical weapons “is a national asset.”

While all branches of the military train against the use of chemical weapons, the U.S. Army Chemical Corps' history of dealing with chemical weapons dates back to World War I. And the Army’s famed 82nd Airborne Division can be used for “vertical envelopment” to parachute to targets, Pike said. “If you wanted to take a crack at putting 4,000 troops on the ground with 24 hours, that would be a good place to start,” he said. A recent 82nd Airborne Division training exercise involved a scenario of seizing chemical weapons in Syria, CBS News reported. “Every soldier in that area would have a chemical [protection] suit -- or ought to,” Pike said. “The thing could fall apart very quickly.”

Destroying chemical weapons take time. Even if they wind up safely under U.S. control in Syria, chemical weapons destruction is a notoriously slow process. The United States has yet to destroy all of its banned chemical stocks, long overdue under the Chemical Weapons Convention. The Army has played a major role in these efforts. But Samore notes that “there’s a big difference in doing it [in the United States] and doing it in Syria in the midst of a civil war.”

All these thorny issues may explain why the Obama administration is not rushing to take military action after publicly determining this spring the Assad regime used chemical weapons against rebels -- and crossing President Obama’s declared “red line” of Syria using or transferring chemical weapons.

The current discussions in Syria focus on whether inspectors can even get proper access to investigate countercharges that both the government and rebels used chemical weapons. So a lengthy U.N. investigative process could provide diplomatic cover against any call for U.S. action to vigorously enforce Obama’s red line.

Absent an unexpected development, always a real possibility in volatile Syria, there is no expectation and seemingly little U.S. desire for immediate action against Syrian chemical weapons. “It’s fine for the 82nd Airborne to practice these things, but I just don’t see the scenario yet,” former Defense Department official Philip Coyle said.

A major fear is terrorist groups may wind up with chemical weapons. “Our inability to fully control Syria’s storage and delivery systems could allow extremists to gain better access,” Dempsey warned.

There’s also great concern that Syrian President Bashar al Assad may be ruthless enough to resort to chemical weapons use as a Doomsday scenario. “You might precipitate use if you launch an attack that is incomplete or unsuccessful,” warned Samore, and “the Syrian government believes there is no reason not use everything they have.”

While planning continues, the potential for deadly consequences greatly dampens the U.S. appetite for seizing Syria’s chemical weapons. “It’s a military operation,” Samore said, “which I think our military hopes they never have to carry out.”