The story goes that after the Bay of Pigs disaster, President Kennedy placed a call to CIA director Allen Dulles. Kennedy supposedly said: "If we lived in a parliamentary system, and I were prime minister, after a disaster of this magnitude I'd have to resign. But we don't. So you do."

Probably the story has been improved on over the years. But there's a core of truth there, which is that there is a lot of fiction in the idea of presidential leadership.

The president of the United States is often described as the world's most powerful man. It seldom feels that way to him. A lot of the reality of the presidency is summed up by that amazing White House Situation Room photograph of the senior national security staff tensely watching the Sunday attack on the bin Laden compound.

Look at President Obama's face. Tense. Anxious. The face of a man who knows his presidency is riding on events unfolding on the other side of the planet, which he can no longer shape or guide.

He has made a few key decisions. He decided in favor of a commando raid against a bombing attack. He decided to disregard the presence of women and children. He decided that bin Laden should be killed rather than captured for trial.

Beyond that, his role was mostly to accept operational decisions made by appropriate professionals. But of course he is not a professional himself. When the question is posed: four helicopters or five? What kind of explosive charge to use to stun the residents at the start of the raid? What measures to counter-act Pakistani radar? Or even -- how much confidence can we have that bin Laden is actually there? To those questions, a president (any president) brings only the instincts, the experience, and the expertise of the career politician. That's what he is, after all.

There is however one other thing a president brings to any major national decision: the knowledge that if things go wrong, it is he who will receive the blame.

This rule of is not absolute. An especially skillful politician can sometimes escape blame, as Ronald Reagan did after the Marine barracks bombing in Beirut. But the more normal outcome is what happened to Jimmy Carter after the humiliating hostage rescue attempt in Iran or what happened to Bill Clinton after the "Blackhawk Down" disaster in Mogadishu: Ferocious criticism at a minimum, re-election defeat at a maximum. If it's bad, we blame the man at the top.

And that's the answer to those who question why Barack Obama should get any credit for a mission that succeeded because of the planning decisions of senior commanders and the courage, skill and marksmanship of Special Forces and CIA professionals whose names we may never know. It's very rough justice, but that's the only kind of justice presidents typically can expect.