The United States (plaintiff) brought suit against Tse (defendant) for distributing cocaine. Tse was convicted. Tse sold cocaine to Stephen Williams, a cooperating DEA witness. The DEA was unable to record the sale, so it called Williams to testify at trial. Tse attempted to impeach Williams by, among other things, proffering evidence of Williams’ prior conviction for assault and battery against a police officer (ABPO). Tse also had a prior conviction for ABPO and the prosecution sought to admit that into evidence. The district court excluded both ABPO convictions, concluding that the analysis of one was closely related to the other. The district court did, however, admit evidence of another crime Williams committed, as well as evidence of his prior involvement with drugs. The district court convicted Tse. He appealed.