“The president believed that it was important to enhance the legitimacy of any action that would be taken by the executive,” Ms. Ruemmler said, “to seek Congressional approval of that action and have it be seen, again as a matter of legitimacy both domestically and internationally, that there was a unified American response to the horrendous violation of the international norm against chemical weapons use.”

At a news conference last week, Mr. Obama argued that the United States should “get out of the habit” of having the president “stretch the boundaries of his authority as far as he can” while lawmakers “snipe” from the sidelines. But he also explained his decision in terms of very special circumstances: humanitarian interventions where there is no immediate pressure to act and the United Nations is blocked.

Jack Goldsmith, a head of the Office of Legal Counsel in the Bush administration, said the limited criteria cited by Mr. Obama mean his move might not apply to more traditional future interventions. The more important precedent, he said, may concern international law and what he portrayed as Mr. Obama’s dismissive attitude toward whether or not having permission from the Security Council should stop humanitarian interventions.

Mr. Obama has in recent days repeatedly portrayed the Security Council system as incapable of performing its function of “enforcing international norms and international law,” and as so paralyzed by the veto power wielded by Russia that it is instead acting as a “barrier” to that goal.

Mr. Goldsmith said that in the Kosovo campaign, the Clinton administration shied away from arguing that it was consistent with international law to carry out a military attack not authorized by the Security Council purely for humanitarian reasons. Its fear was that such a doctrine could be misused by other nations, loosening constraints on war.

In his 2009 Nobel Peace Prize acceptance speech, Mr. Obama said all nations “must adhere to standards that govern the use of force.” But he also argued that humanitarian grounds justified military force and cited “the Balkans,” leaving ambiguous whether he meant Bosnia, which had some Security Council approval; Kosovo, which did not; or both.

Ms. Ruemmler said that while an attack on Syria “may not fit under a traditionally recognized legal basis under international law,” the administration believed that given the novel factors and circumstances, such an action would nevertheless be “justified and legitimate under international law” and so not prohibited.