"When the United States of America says 'Never again,' we don't mean sometimes, somewhere. Never means never," Kerry said. "This is a vote for accountability. Norms and laws to keep a civilized world civil mean nothing if they are not enforced."

Kerry, Defense Secretary Chuck Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff — all three combat veterans — said the intelligence tracing the chemical weapons attack in late August to Assad left little room for doubt. Kerry cited the fact that not a single rocket fell on regime-controlled territory; that regime forces were instructed to wear gas masks in advance; and that the scenes of victims with "no wounds, no blood, but all dead" pointed in that direction.

The conversation may have reminded some lawmakers of 2002, when Bush administration officials descended on Capitol Hill to argue that the administration had evidence that the regime of President Saddam Hussein had weapons of mass destruction — an assertion eventually proven wrong. At the time, Secretary of State Condoleeza Rice told CNN, "We don't want the smoking gun to be a mushroom cloud."

Kerry argued repeatedly that the circumstances were drastically different from a decade ago.

"I remember Iraq, Secretary Hagel remembers Iraq, and General Dempsey remembers Iraq," Kerry said. "We are especially sensitive to never again ask any member of Congress to take a vote on faulty intelligence."

Many senators, even those that expressed support for limited strikes, pushed for more details on why strikes were appropriate at the current moment, when the body count had reached over 100,000 in Syria before the chemical attack, and how military engagement would lead to a desirable resolution. The tone of the hearing was generally polite, if at times tense.

The Wall Street Journal reported Tuesday that Senate Majority Leader Harry Reid, D-Nev., believed he had enough votes to get the resolution through the chamber next week.

None of the Obama administration officials could concretely predict what the outcome of military engagement in Syria would be. They argued that chemical weapons proliferation and more showdowns with despotic dictators would assuredly be the consequence of not intervening.

"There are always unintended consequences of conflicts," Dempsey said.