So much could have gone wrong as SEAL Team Six swept over Pakistan's dark landscape, dropped down ropes into a compound lined by wall after wall, exchanged gunfire and confronted "Geronimo" face to face. The vital things went right.
Just about every contingency the 25 commandos trained for came at them, rapidly, chaotically and dangerously, in their lunge for Osama bin Laden.
They had acted on the best intelligence the U.S. had ever had on bin Laden's whereabouts since he slipped away in the mountains of Tora Bora just under a decade ago. But it was guesswork, too, with the commandos' lives, a president's reputation and a nation's prestige riding on the outcome.
Was the man once seen pacing the compound's courtyard really bin Laden, as it appeared to American eyes? That was just one unknown.
In short, the U.S. had no direct evidence that bin Laden would be there during the assault - or indeed had ever been there. Obama put the raiders in motion on the "pretty good chance" they would find their man, as CIA Director Leon Panetta, who was overseeing the operation back in Washington, put it.