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At the United Nations on Wednesday, President Obama offered a powerful case for war against the Islamic State. "This group has terrorized all who they come across in Iraq and Syria," he said. "There can be no reasoning — no negotiation — with this brand of evil. The only language understood by killers like this is the language of force."

On moral grounds, Mr. Obama's argument is irrefutable. Unfortunately, his legal justification for U.S. air attacks in Iraq and Syria is weaker. The administration submitted a letter to the United Nations saying that it was entitled to act under the "collective right of self-defense," because the government of Iraq had asked the United States to "lead international efforts to strike [Islamic State] sites and military strongholds in Syria" in order to defend Iraq. Separately, Mr. Obama informed Congress that he considered his action authorized by the Constitution's grant of war-making authority to the president and by the resolutions Congress passed in 2001 and 2002 authorizing military action against al-Qaida and Iraq.

Legal experts contest both of Mr. Obama's assertions. They point out that neither the U.N. Security Council nor the Syrian government have authorized U.S. military action in Syria; the administration's contention that Syria is "unwilling or unable" to stop Islamic State attacks on Iraq is not universally accepted as a principle of international law. Separately, the U.S. letter says that strikes against the Khorasan terror group in Syria are based on "the terrorist threats they pose to the United States and our partners and allies"; but the administration has not released detailed evidence documenting its claim that Khorasan was preparing attacks.

As a practical matter, U.S. allies and most other nations are unlikely to challenge the war, while Russia is likely to ensure that no authorizing resolution clears the Security Council.

It helps that U.N. Secretary General Ban Ki-moon was supportive of the administration's rationale, saying, "I think it is undeniable — and the subject of broad international consensus — that these extremist groups pose an immediate threat to international peace and security."

Domestically, too, Mr. Obama's legal case probably won't be seriously contested by Congress or the courts. That, however, doesn't mean that the president's legal rationale is satisfying. Numerous experts have pointed out that, in citing the 2001 Authorization for the Use of Military Force against al-Qaida, Mr. Obama is using a 13-year-old statute that he himself has said should be narrowed and eventually repealed. Though the law authorizes action against "associated forces" of al-Qaida, the White House is applying it to a group that split with al-Qaida and has been repudiated by it.

Especially as Mr. Obama is describing the war against the Islamic State as one that may last for years, only passage by Congress of a new authorization will provide an unassailable mandate. Half a dozen proposed resolutions have been introduced; two come from Democrats Rep. Adam Schiff, Calif., and Sen. Tim Kaine, Va. They are too narrow in limiting military action: Mr. Kaine's bill would prevent the use of U.S. ground forces that senior military commanders say may be needed.

Congress should not preempt decisions on how to win the war. But Mr. Kaine and Mr. Schiff are right to challenge Congress to make itself accountable for the new intervention in the Middle East, and provide Mr. Obama with clear legal and political grounding.