The soldiers at the blast crater sensed something was wrong. It was August 2008 near Taji, Iraq. They had just exploded a stack of old artillery shells buried beside a murky lake. The blast, part of an effort to destroy munitions that could be used in makeshift bombs, uncovered more shells. Two technicians assigned to dispose of munitions stepped into the hole. Lake water seeped in. One of them, Specialist Andrew T. Goldman, noticed a pungent odor, something, he said, he had never smelled before. He lifted a shell. Oily paste oozed from a crack. “That doesn’t look like pond water,” said his team leader, Staff Sergeant Eric J. Duling. The specialist swabbed the shell with chemical-detection paper. It turned red - indicating sulfur mustard, the chemical warfare agent designed to burn a victim’s airway, skin and eyes. All three men recall an awkward pause. Then Sergeant Duling gave an order: “Get the hell out.” Five years after President George W. Bush sent troops into Iraq, these soldiers had entered an expansive but largely secret chapter of America’s long and bitter involvement in Iraq. From 2004 to 2011, American and American-trained Iraqi troops repeatedly encountered, and on at least six separate occasions were wounded by, aged chemical weapons remaining from the time of Saddam Hussein’s rule. In all, American troops secretly reported finding more than 4,990 chemical munitions, according to interviews with dozens of participants, Iraqi and American officials and to heavily redacted intelligence documents obtained under the Freedom of Information Act. The United States had gone to war to destroy an active weapons of mass destruction program. Instead American troops gradually found and ultimately suffered from the remnants of a long abandoned program, built in close collaboration with the West. Those findings were neither publicly shared nor widely circulated within the military, and carry potentially ominous implications now that the Islamic State, a Qaeda splinter group, controls much of the territory where the chemical weapons were found. The American government withheld word about its chemical-weapon discoveries even from troops it sent into harm’s way and military doctors who were to organize victims’ care. The government’s secrecy, victims and participants said, prevented troops in some of the war’s most dangerous jobs from receiving proper medical treatment or official recognition. “I felt more like a guinea pig than a wounded soldier,” said a former Army sergeant who suffered mustard burns in 2007; this soldier’s commander said the wounded sergeant was denied hospital treatment and medical evacuation to the United States. Congress too was misled, while troops and officers were instructed to be silent or give deceptive accounts of what they had found. ”‘Nothing of significance’ is what I was ordered to say,” said Army Major Jarrod Lampier, who was present for the largest chemical weapons discovery of the war: 2,400 nerve-agent rockets unearthed in 2006 on a former Republican Guard compound. Jarrod L. Taylor, a former Army sergeant on-hand for the destruction of mustard shells that burned two soldiers in his infantry company, joked of “wounds that never happened” from “that stuff that didn’t exist.” The public, he said, had been misinformed for a decade. “I love it when I hear “Oh there weren’t any chemical weapons in Iraq,” he said. “There were plenty.” The discoveries of these weapons, several officials and participants in the recoveries said, did not support the government’s pre-invasion rationale for the war. After the terrorist attacks on Sept. 11, 2001, President Bush insisted that that Mr. Hussein was hiding an active weapons-of-mass-destruction program, in defiance of international will and at risk to the world. United Nations inspectors said they could not find evidence for these claims. Then, during the period of occupation, American troops began finding chemical munitions in hidden caches and roadside bombs. Typically 155-millimeter artillery shells or 122-millimeter rockets, they were remnants of an arms program Iraq had rushed into production in the 1980s during the Iran-Iraq War. All had been manufactured before 1991, participants said. Almost all could not have been used as designed, according to troops and officers who collected the majority of them. Filthy, rusty or corroded, a large fraction of them could not be readily identified as chemical weapons at all, though many of them still contained potent mustard agent or residual sarin. In case after case, participants said, analysis of the warheads and shells reaffirmed American intelligence failures. First, American government did not find what it had been looking for at the war’s outset, then it failed to prepare its troops and medical corps for the perils posed by the aged weapons it did find. As Iraq has been shaken anew by violence, and past security gains have collapsed amid Sunni-Shiite bloodletting and the rise of the Islamic State, this long-hidden chronicle illuminates the persistent menace of the country’s abandoned chemical weapons. Many chemical-weapons incidents clustered around the ruins of the Al Muthanna State Establishment, the center of Iraqi chemical agent production in the 1980s. Since June, the compound has been held by the Islamic State, the world’s most radical and violent jihadist group. In a letter sent to the United Nations this summer, the Iraqi government said about 2,500 corroded chemical rockets remained on the grounds, and Iraqi officials had witnessed intruders looting equipment before militants shut the surveillance cameras down. The United States government says these weapons no longer pose a threat. But the near decade of wartime experience showed that old Iraqi chemical munitions often remained dangerous when repurposed in makeshift bombs, as insurgents did repeatedly starting in 2004. Participants in the chemical-weapons discoveries said the United States suppressed knowledge of its finds for multiple reasons, including that the government bristled at further acknowledgement it had been wrong. “They needed something to say that after Sept. 11 Saddam used chemical rounds,’’ Major Lampier said. “And all of this was from the pre-1991 era.” Others pointed to another embarrassment. In five of six incidents in which troops were wounded by chemical agents, the munitions appeared to have been designed in the United States, manufactured in Europe and filled in chemical-agent production lines built in Iraq by Western firms. Non-proliferation officials said the Pentagon’s activities risked putting the United States in non-compliance of the Convention on Chemical Weapons. According to this convention, chemical weapons must be secured, reported on and destroyed in an exacting and time-consuming fashion. The Pentagon insists that it followed the convention’s spirit. “These suspect weapons were recovered under circumstances in which prompt destruction was dictated by the need to ensure that the chemical weapons could not threaten the Iraqi people, neighboring states, coalition forces, or the environment,” said Jennifer Elzea, a Pentagon spokeswoman. The convention, she added, “did not envisage the conditions found in Iraq by U.S. forces.” Nonetheless, several participants said the United States lost track of chemical weapons that its troops found, left at least three large caches unsecured, and did not warn people - Iraqis and foreign troops alike - as it hastily exploded chemical ordnance in the open air. This was the secret world that Sergeant Duling and his soldiers were entering as they stood above the leaking chemical shell. The sergeant spoke into a radio, warning everyone back. “This is mustard agent,’’ he said, announcing the beginning of a journey of inadequate medical care and honors denied. “We’ve all been exposed.” CONT...