Rosemond v. United States, ___ U.S. ___ (2014) (discussion of elements of aiding and abetting).

The court stated:

When an accomplice knows beforehand of a confederates design to carry a gun, he can attempt to alter that plan or, if unsuccessful, withdraw from the enterprise; it is deciding instead to go ahead with his role in the venture that shows his intent to aid an armed offense. But when an accomplice knows nothing of a gun until it appears at the scene, he may already have completed his acts of assistance; or even if not, he may at that late point have no realistic opportunity to quit the crime. And when that is so, the defendant has not shown the requisite intent to assist a crime involving a gun. As even the Government concedes, an unarmed accomplice cannot aid and abet a 924(c) violation unless he has foreknowledge that his confederate will commit the offense with a firearm. Brief for United States 38; see also infra, at 15"17. For the reasons just given, we think that means knowledge at a time the accomplice can do something with it"most notably, opt to walk away.

Id. at ___.