The White House on Tuesday began to lay out a public justification for a possible bombing of Syria, saying the nation’s use of chemical weapons is a threat to U.S. interests — a scenario that opens the door to military strikes that wouldn’t require authorization from Congress or the United Nations. …

“It is in the clear national security interests of the United States that the use or proliferation of chemical weapons on this scale not go unanswered,” Mr. Carney said, adding that the attack was of particular concern because it took place “in this highly volatile region.”

Mr. Carney did not describe the intelligence that linked Syrian President Bashar Assad to the attack or how it was a direct threat to the U.S., which would give the Mr. Obama latitude in starting a bombing campaign. But he did say the Assad regime is the only force in Syria known to have chemical weapons and the rockets that could deliver them. …

“Before any action is taken regarding Syria, it is imperative that President Obama make the case to the American people and consult with Congress,” said Sen. John Cornyn of Texas, the second-ranking Republican in the Senate. “He needs to explain what vital national interests are at stake and should put forth a detailed plan with clear objectives and an estimated cost for achieving those objectives.”