Nor would a strike be aimed at altering the course of Syria’s two-year-old civil war. White House officials said Mr. Obama remained convinced that intervening more broadly in the conflict would cause more problems in the region than it would solve.

Unlike Iraq in 2003, the triggering event in Syria a decade later is not a shaky argument that the government possesses weapons of mass destruction, but a huge rocket assault that left hundreds of victims convulsing and gasping for breath, glassy-eyed and foaming at the mouth — all classic symptoms of a reaction to poison gas. Video images of the carnage were posted on YouTube, while the aid group Doctors Without Borders issued a report based on firsthand accounts from medics at the overwhelmed hospitals.

“These all strongly indicate that everything these images are already screaming at us is real, that chemical weapons were used in Syria,” Secretary of State John Kerry said on Monday.

Mr. Kerry did not show any of those pictures, unlike his predecessor Colin L. Powell, who in February 2003 displayed satellite photographs, played intercepts of conversations between Iraqi officials, and brandished a vial of white powder in his futile effort to persuade the United Nations Security Council to coalesce behind the invasion of Iraq.

Such an elaborate campaign is not necessary in this case, administration officials argued, because the evidence is already overwhelming and the scope of the proposed response is more limited.

Still, the White House faces an American public considerably more skeptical about intervention in Syria than it was about Iraq. The feverish atmosphere of the years after the Sept. 11 attacks has given way to a country exhausted after more than a decade of war.

Mr. Bush obtained strong Congressional backing for the war in Iraq, but now even the Republican Party is split between hawks like Senator John McCain of Arizona, who advocates forceful intervention, and neo-isolationists like Senator Rand Paul of Kentucky, who argues that the United States has no business getting entangled in the Middle East. At this point the bulk of Republican members of Congress are skeptical of taking military action.