Saddam Hussein didn’t have a weapons of mass destruction program when the US invaded Iraq in 2003. But the country, in many of the same places now controlled by ISIL, had scattered stockpiles of chemical weapons dating back to the Iran-Iraq war. The US has never revealed the full extent of these stockpiles—in part because they were made with US designs and assistance.

A New York Times investigation discovered that at least 17 US troops (paywall), many in bomb disposal units, were exposed to mustard and sarin gas as they defused improvised explosive devices, the conflict’s signature weapon, and destroyed Iraqi munitions stockpiles. As they suffered from blisters and breathing problems, their medical treatment was frequently lax, Times reporter C. J. Chivers reports, and the US military covered up their discoveries, preventing troops and physicians for preparing for the threat.

One reason for the cover-up is the US connection to the weapons’ origins. The West backed Hussein’s Iraq in its war with Iran, choosing a secular dictatorship over the Iranian theocracy. European firms in Italy and Germany used American designs to build artillery shells for use with chemical weapons, selling at least 85,000. A German firm built a massive chemical weapons facility in Iraq, and two American companies provided the chemicals needed to produce mustard gas.

And now ISIL has seized that territory, and perhaps some of those munitions with it. Sketchy reports suggest that the extremist group may have already deployed chemical weapons of unknown origin. Even if ISIL’s chemical weapons access isn’t “militarily significant” (to borrow from a US assessment of the scattered chemical munitions in Iraq) the possibility adds an extra sheen of terror to the group’s actions and complicates plans for military intervention against them.

And, as Western nations ramp up efforts to arm secular fighters against an Islamist movement—just as they did in the 1980s—these revelations are also a reminder that sometimes such enterprises come with unintended consequences.