An administration document on the use of chemical weapons since World War I was also part of a briefing provided to the White House around Syrian President Bashar al-Assad's chemical attack, sources with knowledge of the briefing told CNN.
The briefing document, one source said, examined "the history of battlefield uses of chemical weapons" that was meant to "demonstrate the declining acceptability of these weapons." The document showed the "trend line of acceptability of use of these weapons" and how they are only used by "the worst outliers."
The document noted how Germany and others used chemical weapons on the battlefield in World War I, but did not use them in World War II.
After authorizing strikes in response to Assad's use of chemical weapons on his own people, Trump was plagued by a question: Why would a world leader use chemical weapons to kill their own people?
According to aides and people who spoke to Trump Friday, the President was ruminating about the use of chemical weapons in a broader historical and philosophical context and asked his aides and close confidantes for answers.
The President, still reeling from images of Syrians killed in the chemical attack which aides said pushed him to act against Assad, couldn't seem to reconcile why a leader would resort to using chemical weapons on his own people, said one person who spoke with Trump, given the implications of the attack.
The fact that this question plagued Trump is significant, and could explain why Trump has so far called Assad "evil," "an animal," and, most recently, "a butcher."
The focus -- by both Trump and his aides -- could also explain how a trio of top White House officials, including Trump, spoke about chemical weapons in a historical context on Tuesday.
White House press secretary Sean Spicer incorrectly diminished Nazi leader Adolf Hitler's use of chemical weapons in World War II. The comment -- and subsequent efforts to clarify -- led Jewish groups and Democrats to call for Spicer's job.
"You had someone who was despicable as Hitler who didn't even sink to using chemical weapons," Spicer said during his press briefing Tuesday. "So you have to, if you are Russia, ask yourself is this a country and a regime that you want to align yourself with."
Spicer later apologized
in an interview with CNN's Wolf Blitzer.
"I was obviously trying to make a point about the heinous acts that Assad had made against his own people last week, using chemical weapons and gas. Frankly, I mistakenly made an inappropriate and insensitive reference to the Holocaust, for which there is no comparison," Spicer said Tuesday. "And for that I apologize. It was a mistake to do that."
A White House official flatly denied that the briefing document led to Spicer's comments. Spicer did not respond to questions about the document.
But also on Tuesday, Trump Defense Secretary James Mattis brought up the historical comparisons.
"The intent was to stop the cycle of violence into an area that even in World War II chemical weapons were not used on battlefields," Mattis told reporters. "Even in the Korean War, they were not used on battlefields. Since World War I, there has been an international convention on this and to stand idly by when the convention is violated -- that is what we had to take action on recently in our own vital interest."
Trump, in an interview with Fox Business on Tuesday, said "even some of the worst tyrants in the world didn't use the kind of gases that they used."