WASHINGTON — When Syria shipped what it claimed was the last of its chemical weapons out of the country in 2014, John Kerry, the secretary of state at the time, declared that it showed that skillful diplomacy could achieve far more than attacks on a few facilities.

“We struck a deal where we got 100 percent of the chemical weapons out,” he said a few weeks later, as an American ship destroyed 600 metric tons of poisonous agents.

A year ago, after President Trump rejected the Obama-era approach as naïve, he bombed an airfield where a new chemical attack by the Syrian president, Bashar al-Assad, had originated. Mr. Trump’s newly appointed national security adviser, Lt. Gen. H. R. McMaster, predicted “a big shift on Assad’s calculus,” because it was “the first time the United States has taken direct military action.”

Years of bitter experience in Syria have shown that Mr. Kerry’s assessment was wrong, and General McMaster’s was far too optimistic. Those lessons may now be inescapable: After Saturday’s predawn strike in Syria on three suspected chemical weapons sites, government officials and outside experts agreed that the attack, while double the size of last year’s, was unlikely to eliminate Mr. Assad’s ability to gas his own people yet again.