WASHINGTON — When the Syrian government carried out a gruesome chemical attack on civilians this week, many people had a question: Didn’t the Obama administration, working with Russia and international experts, destroy Syria’s chemical weapons stocks in 2014?

In his State of the Union address that year, President Barack Obama declared, “American diplomacy, backed by the threat of force, is why Syria’s chemical weapons are being eliminated.” Months later, in July, on NBC’s “Meet the Press,” Secretary of State John Kerry essentially declared the mission accomplished: “We struck a deal where we got 100 percent of the chemical weapons out.”

But, as became obvious when a Syrian attack on Tuesday killed more than 80 people, the truth was more complicated. Here is a primer on the history of Syria’s chemical stockpile, the effort to eliminate it and experts’ views on the new attack.

When did Syrian forces first use chemical weapons, and how did the United States respond?

Scattered reports of chemical attacks have been made since the beginning of the Syrian civil war in 2011, but a large-scale attack in August 2013 — with United Nations inspectors already on the ground — got the world’s attention. Mr. Obama said he intended to carry out a limited military strike to uphold the international ban on chemical weapons and deter further attacks. Then he decided to seek authorization from Congress first.