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.

From The New York Times:

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?

“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?”

The Oval Office meeting ended one of the strangest weeks of the Obama administration, in which a president who had drawn a “red line” against the use of chemical weapons, and watched Syrian military forces breach it with horrific consequences, found himself compelled to act by his own statements. But Mr. Obama, who has been reluctant for the past two years to get entangled in Syria, had qualms from the start.

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.

 

It was only on Friday that he told the aides, they said, about how his doubts had grown after the vote: a verdict, Mr. Obama told his staff, that convinced him it was all the more important to get Congressional ratification. After all, he told them, “we similarly have a war-weary public.”
 

And if the British government was unable to persuade lawmakers of the legitimacy of its plan, shouldn’t he submit it to the same litmus test in Congress, even if he had not done so in the case of Libya?
 

Mr. Obama’s backing of a NATO air campaign against Col. Muammar el-Qaddafi in 2011 had left a sour taste among many in Congress, particularly rank-and-file members. More than 140 lawmakers, Republicans and Democrats, had signed a letter demanding a vote on Syria.
 

Moving swiftly in Libya, aides said, was necessary to avert a slaughter of rebels in the eastern city of Benghazi. But that urgency did not exist in this case.