The U.S. knew about, and in one case helped, Iraq's chemical weapons attacks against Iran in the 1980's, according to recently declassified CIA documents obtained by Foreign Policy. Their detailed timeline, also constructed with the aid of interviews with former foreign intelligence officials, indicates that the U.S. secretly had evidence of Iraqi chemical attacks in 1983. The evidence, FP writes, is "tantamount to an official American admission of complicity in some of the most gruesome chemical weapons attacks ever launched."

Ever since last week's devastating evidence of chemical attacks in Syria, analysts have looked for benchmarks to predict the U.S.'s response. On Sunday, a U.S. official suggested that the U.S. is moving closer to possible military action in the country as the U.S. has "little doubt" that an "indiscriminate" chemical attack took place. Officials are reportedly looking to the 1998 air war on Kosovo for a precedent — a similar humanitarian crisis in the face of virtually no chance of a U.N. Security Council resolution to authorize use of force, thanks to dissent from Russia. And while Foreign Policy's additional reporting places the Iraq situation in contrast to today's debate over Syria, the details reveal just how sharply, in the past, the razor of U.S. interests in the Middle East has cut: "it was the express policy of Reagan to ensure an Iraqi victory in the war, whatever the cost," the report explains. And apparently, that went up to and including helping Saddam Hussein gas Iran.

From 1983 until 1987,  the U.S. more or less sat on (and internally discussed) intelligence containing strong evidence of Iraq's chemical weapons use — early on, that meant mustard gas. Retired Air Force Col. Rick Francona told the magazine that he first learned of Iraq's chemical weapons use in 1984. All that time, Iran was publicly saying that Iraq had used chemical weapons against them. They just didn't have any evidence to take to the U.N.  Then, Iran concentrated a large number of troops near the Iraqi city of Barash, near a vulnerability in Iraq's defenses:

In late 1987, the DIA analysts in Francona's shop in Washington wrote a Top Secret Codeword report partially entitled "At The Gates of Basrah," warning that the Iranian 1988 spring offensive was going to be bigger than all previous spring offensives, and this offensive stood a very good chance of breaking through the Iraqi lines and capturing Basrah. The report warned that if Basrah fell, the Iraqi military would collapse and Iran would win the war.

President Reagan read the report and, according to Francona, wrote a note in the margin addressed to Secretary of Defense Frank C. Carlucci: "An Iranian victory is unacceptable."

The U.S. authorized intelligence sharing with Iraq, and gave Iraq the location of those troops. Iraq then conducted a series of devastating sarin gas attacks. You can view all of the declassified documents (or read the whole report) at Foreign Policy.