Under the principle of accomplice liability, you can be convicted of a crime simply for helping someone commit it. As long as you intended to aid in the offense, you’re an aider and abettor. But what, exactly, do you have to know or intend to be convicted as an accomplice?
According to the Supreme Court, in order to commit a federal aiding-and-abetting offense, an accomplice must know someone will commit all elements of a crime, but only has to intend to help with one element.
Example: Avon arranges to sell five kilos of cocaine to Joe. Russell and D’Angelo agree to accompany him to the sale as protection. But once they arrive, one of Joe’s associates swipes the drugs from Avon. Joe and his associates run. D’Angelo takes out a semiautomatic and fires at the robbers, but misses. At a minimum, Russell is guilty of aiding and abetting possession of cocaine for sale. That crime has two elements: (1) possession (2) with intent to sell. Russell knew that Avon both possessed and intended to sell cocaine; he intended to and did help him possess the drugs for sale by serving as protection. (United States v. Poston, 902 F.2d 90 (D.C. Cir. 1990).)
Example: In the above example, the prosecution also charges Russell with aiding and abetting drug possession involving use of a firearm. A conviction for that crime requires proof of two elements: (1) drug possession and (2) firearm use. As long as Russell intended to aid in the drug-possession element, it doesn’t matter whether he intended to aid in the firearm-use element. But he had to at least know that use of a gun was a possibility, and he had to know that early enough to have an opportunity to quit the crime. The prosecution must therefore prove that Russell knew about the gun—at least at some point before D’Angelo used it. (Rosemond v. U.S., 572 U. S. ____ (2014).)
For more on this concept, see Aiders and Abettors, and Drug Deals Gone Bad.