UPDATE: On Saturday afternoon, President Obama said he would seek Congressional authorization before he launches a limited military strike on Syria.

As President Obama moves toward unilateral military action in response to a chemical weapons attack in Syria that killed more than 1,400 people, he is doing so without legal justification and without the backing of two key institutions, Congress and the United Nations Security Council. Both have abdicated their roles in dealing with this crisis.

Secretary of State John Kerry said forcefully on Friday that there was no doubt that the government of President Bashar al-Assad was behind the attack. Both he and President Obama made a largely moral case for a retaliatory response. The administration also argued that failure to respond could lead Mr. Assad, his Hezbollah allies, Iran and North Korea to believe they can violate international norms with impunity. But no administration official has formally asserted a legal basis — absent a vote of Congress or the Security Council — for military strikes.

Congress spends a lot of time jealously guarding its powers, especially when it comes to Republicans thwarting Mr. Obama’s agenda. But apart from complaining, asking questions and getting briefed by administration officials, most senators and representatives seem content to leave this exceedingly difficult decision to President Obama. They should have returned to Washington from summer vacation to debate and vote on the Syria issue. Prime Minister David Cameron of Britain suffered a defeat when Parliament on Thursday voted to oppose involvement in a military operation, but at least the British lawmakers had to step up and take a stand.