If Syrian dictator Bashar Assad decides to use his chemical weapons, there won't be a thing the U.S. military can do to stop him, America's top military officer conceded on Thursday. Nor will the U.S. step into a "hostile" atmosphere, with or without Assad, to keep those chemicals under control.

It's been a month since U.S. intelligence learned that Assad's forces were mixing some of their precursor chemicals for sarin gas, as Danger Room first reported. The Syrian military even loaded aerial bombs with the deadly agent. Assad hasn't used the weapons – yet. Should he change his mind, there's little chance the U.S. would know it before it's too late to stop the first chemical attack in the Mideast in over 20 years.

"The act of preventing the use of chemical weapons would be almost unachievable," Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told reporters at the Pentagon. "You would have to have such clarity of intelligence, persistent surveillance, you'd have to actually see it before it happened. And that's unlikely, to be sure."

That explains the emphasis the Obama administration has given, from President Obama and Defense Secretary Leon Panetta on down, to publicly warning Assad that using his chemical weapons would cross a "red line." Dempsey said that "messaging" seeks to establish a deterrent, since Assad might think it would prompt outright U.S. or international intervention leading to his downfall. But that's different from preemption.

American officials began strategizing months ago for how it should operate in a post-Assad Syria. And that includes scoping out plans for disposing of Assad’s stockpiles of nerve and mustard agents.

Today, however, Panetta shot down a related preventive step: sending U.S. troops into the chaos of the Syrian civil war to secure the chemical stocks.

U.S. military officials have previously speculated that an intervention to take hold of an estimated 500 tons of chemical precursors would require 75,000 troops, a force larger than the one currently in Afghanistan. Panetta said the international community needs to establish a "process and procedure" for keeping the stockpiles under control – but only after Assad falls, which is an uncertain proposition. U.S. intervention to lock down the chemicals, Panetta said, would depend on the establishment of new regime willing to invite the U.S. military in – another uncertain proposition.

"We're not working on options that involve boots on the ground," Panetta said. If there's a "peaceful transition," then the U.S. might consider a request that a friendly successor government might make to secure the chemical stocks. "But in a hostile situation, we're not planning for that." It's looking likely that the 400 U.S. soldiers sent to Turkey to man Patriot missile batteries could be the only uniformed troops that the Pentagon openly sends to handle the Syrian crisis.

The U.S. public has little appetite for throwing exhausted U.S. soldiers and marines into yet another bloody Mideastern conflict. But Panetta and Dempsey's concession underscores the massive risks that the Syrian civil war poses for either the use or black market proliferation of chemical weapons. The revolution has already claimed the lives of 60,000 Syrians. The longer it goes on, the greater the pressure Assad may feel to unleash his unconventional arms. Alternatively, various Syrian factions might be either unwilling or unable to secure the stocks, should they prevail, nor is there any guarantee they will give up the chemical weapons once victorious.

There is confusion about how long the sarin gas will remain usable once its precursors combine. Nerve agents are inherently unstable, but U.S. government sources have told Danger Room that Syrian sophistication with chemical weaponry may leave the combined, weaponized sarin deadly for up to a year. Dempsey and Panetta, however, believe that they'll break down after 60 days. "That's what the scientists tell us," Dempsey said. "I'd still be reluctant to handle it myself."

Disposing of (or "demilitarizing") chemical weapons is extraordinarily difficult under any circumstances; Iraq’s former chemical bunkers are still toxic nearly than a decade after Saddam’s overthrow, and the U.S. recently said it won’t be done disposing of its Cold War chemical weapon arsenal until 2023. Assad’s nerve agents will be no exception.

One of sarin's main precursors – methylphosphonyl difluoride, or DF – can be turned into a somewhat non-toxic slurry, if combined properly with lye and water. The problem is that when DF reacts with water, it generates heat. And since DF has an extremely low boiling point — just 55.4 degrees Celsius — it means that the chances of accidentally releasing toxic gases are really high. "You could easily kill yourself during the demil," one observer told Danger Room during the fall. That would explain Dempsey's reluctance to touch it.

Naturally, this process could only begin once the DF and the rubbing alcohol (sarin's other main precursor) was gathered up from Assad’s couple dozen storage locations. Then, they’d have to be carted far, far out into the desert — to make sure no bystanders could be hurt — along with the enormous stirred-tank reactors needed to conduct the dangerous chemistry experiments. And when it was all done, there would the result would be a whole lot of hydrofluoric acid, which is itself a poison.

It's an operation that will take many months, many men, and many millions of dollars. No wonder the leaders of America's overtaxed military won't commit to the job until the Syrian civil war is done.