WASHINGTON — The White House press secretary, Sean Spicer, set off an intense backlash on Tuesday when he suggested that President Bashar al-Assad of Syria was guilty of acts worse than Hitler and asserted that Hitler had not used chemical weapons, ignoring the use of gas chambers at concentration camps during the Holocaust. Mr. Spicer later apologized.

During his daily briefing for reporters, Mr. Spicer was defending President Trump’s decision to order a missile strike on Syria by trying to lend gravity to the actions of Mr. Assad. American officials accuse the Syrian president of using sarin gas, a lethal chemical weapon, in an attack on a rebel-held area of Idlib Province last week that killed dozens, many of them children.

But in misconstruing the facts of the Holocaust — Nazi Germany’s brutally efficient, carefully orchestrated extermination of six million Jews and others — Mr. Spicer instead drew a torrent of criticism and added to the perception that the Trump White House lacks sensitivity and has a tenuous grasp of history.

“We didn’t use chemical weapons in World War II,” Mr. Spicer said. “You know, you had someone as despicable as Hitler who didn’t even sink to using chemical weapons.”