100 years later the United States is still struggling to get rid of its chemical weapons. Some 780,000 mustard gas shells are stored near a Colorado disposal plant, the vast majority of the remaining U.S. stockpile. The plant isn't even operational yet - that starts this October. Destroying the stockpile will take at least four years and cost a minimum of $4.5 billion.
This is the legacy of one of mankind's most vile inventions: chemical weapons.
"Chemical agent destruction is a hard role. It's a high hazard operation," says project manager Kim Jackson.
At a cutting edge military installation on the windswept prairie of southern Colorado, an army of workers in protective gear, assisted by precision robots, is training to destroy the toxic and increasingly unstable weapons. The facility's official name is the Pueblo Chemical Agent Destruction Pilot Plant.
"We have explosion hazards, and we also have agent hazards. That means anyone who might be exposed to the blister agent. So we spend a lot of our time with our personnel on training to ensure that our workforce is ready to complete chemical weapons destruction." says Jackson.
In the 1940s, as the US entered World War Two, the military created a huge stockpile of chemical weapons, mainly in the form of artillery shells.
Fortunately for humanity, neither side used chemical weapons against enemy troops during the war. So the ageing munitions - primarily mustard gas - sat for decades inside concrete and earth covered bunkers at the Pueblo Chemical Depot. The weapons date to as far back as the first world war.
In the 1990s, the U.S. and many other nations signed a treaty pledging to destroy their stockpiles. After much delay that process is happening now to the mustard gas shells stored at Pueblo.
"Mustard rounds were introduced in World War I," Jackson said. "And to think here we are 100 years later completing destruction of those chemical weapons."
Right now the workers are practicing on dummy shells exactly like the artillery that was manufactured during World War II.
Full scale weapons destruction will begin in October. Currently, Jackson is running a battery of tests on both equipment and personnel.
"They never know what I'm going to throw at them," she says, smiling. "We use different contingencies, such as a medical event, a chemical spill, or a loss of equipment, and they have to respond."
When the process begins for real each shell will be carefully unpacked and have its explosives removed, then repeatedly checked for leaks. The round will then be taken apart, soaked in neutralizing chemicals, blasted with high pressure water, and baked in ovens to strip away every trace of poison.
In a more dangerous and separate building, some live shells that are leaking or have been damaged are already being destroyed with controlled explosions.
Workers clad in protective clothing carefully load rounds into a thick steel cylinder, constantly checking items off a lengthy safety checklist. Inside the cylinder, explosive charges neatly split the shells in two, which are then treated with neutralizing agents.
"The mustard gas is inside the destruction system vessel. That's neutralized with monoethanolamine," a spokesman says. "We rotate the vessel and typically in no more than an hour the monoethanolamine has completely broken down the mustard agent."
While the engineers who carry out this dangerous and painstaking job don't usually talk about the historical or moral aspects of their work, engineer Jon Miller sees it is as a deeply satisfying task.
"Chemical weapons are about the worst thing going," Miller says. "They're dirty, they're nasty. So really, getting rid of them, in my opinion, is kind of an important thing."