According to this Foreign Policy report, which is based on "recently declassified CIA documents," American officials knew Saddam Hussein was going to use chemical weapons in his 1980s war with Iran—and helped him do it:

In 1988, during the waning days of Iraq's war with Iran, the United States learned through satellite imagery that Iran was about to gain a major strategic advantage by exploiting a hole in Iraqi defenses. U.S. intelligence officials conveyed the location of the Iranian troops to Iraq, fully aware that Hussein's military would attack with chemical weapons, including sarin, a lethal nerve agent.

The intelligence included imagery and maps about Iranian troop movements, as well as the locations of Iranian logistics facilities and details about Iranian air defenses. The Iraqis used mustard gas and sarin prior to four major offensives in early 1988 that relied on U.S. satellite imagery, maps, and other intelligence. These attacks helped to tilt the war in Iraq's favor and bring Iran to the negotiating table, and they ensured that the Reagan administration's long-standing policy of securing an Iraqi victory would succeed. But they were also the last in a series of chemical strikes stretching back several years that the Reagan administration knew about and didn't disclose.

However, that was more than two decades ago. In 1988, Barack Obama was in law school, YouTube didn't exist, and the Chemical Weapons Convention—to which Syria is not a signatory—didn't exist, either.