Syrian chemical weapons personnel under the chain of command of the Syrian Ministry of Defense prepared for three days before Syrian military units, instructed to don gas masks, fired rockets containing nerve gas into opposition-held villages in the Damascus suburbs on August 21, killing over 1,400 people in the largest chemical weapons attack in a quarter century, the United States said Friday.

"The primary question is really no longer: What do we know?" Secretary of State John Kerry said in a powerful call to action from the State Department Friday, as the US released a declassified, four-page intelligence assessment on the attack. "The question is: What are...we in the world going to do about it?"

"History would judge us all extraordinarily harshly if we turned a blind eye to a dictator’s wanton use of weapons of mass destruction against all warnings, against all common understanding of decency," Kerry warned.

The release of the US intelligence case (map) and Kerry's call for resolve came after President Obama met again with his national security cabinet Friday morning on Syria, amid daily intensified preparations for almost certain US-led military strikes in Syria in the coming days. But the Obama administration suffered a blow when the British parliament voted early Friday against UK participation in any international Syria action. France, however said Friday, it would participate, and was sending two frigates to the area where the US has already sent five warships.