He had several reasons, he told them, including a sense of isolation after the terrible setback in the British Parliament. But the most compelling one may have been that acting alone would undercut him if in the next three years he needed Congressional authority for his next military confrontation in the Middle East, perhaps with Iran.

If he made the decision to strike Syria without Congress now, he said, would he get Congress when he really needed it?

“He can’t make these decisions divorced from the American public and from Congress,” said a senior aide, who spoke on the condition of anonymity to discuss the deliberations. “Who knows what we’re going to face in the next three and a half years in the Middle East?” …

Even as he steeled himself for an attack this past week, two advisers said, he nurtured doubts about the political and legal justification for action, given that the United Nations Security Council had refused to bless a military strike that he had not put before Congress. A drumbeat of lawmakers demanding a vote added to the sense that he could be out on a limb.