WASHINGTON — President Obama’s aides were stunned at what their boss had to say when he summoned them to the Oval Office on Friday at 7 p.m., on the eve of what they believed could be a weekend when American missiles streaked again across the Middle East.

In a two-hour meeting of passionate, sharp debate in the Oval Office, he told them that after a frantic week in which he seemed to be rushing toward a military attack on Syria, he wanted to pull back and seek Congressional approval first.

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?