Military Rule of Evidence 404(b) permits introducing evidence of prior crimes, wrongs, or acts for specific non-propensity purposes including proving motive, opportunity, intent, preparation, plan, knowledge, identity, absence of mistake, or lack of accident. The military judge applies a three-part test: (1) the evidence must be offered for a proper purpose; (2) it must be relevant to that purpose; and (3) its probative value must substantially outweigh unfair prejudice under MRE 403.
The prosecution must provide reasonable notice of intent to use 404(b) evidence, articulating the specific non-propensity purpose. Common applications include showing pattern evidence in sexual assault cases, demonstrating knowledge in drug distribution prosecutions, or proving intent through similar prior conduct. The evidence must make a fact of consequence more or less probable without relying on propensity reasoning. Temporal proximity, similarity of acts, and strength of proof affect admissibility.
Military judges must provide limiting instructions explaining proper use of such evidence, emphasizing it cannot prove character or propensity. The defense can challenge admissibility by arguing improper purpose, lack of relevance to stated purpose, or that prejudice substantially outweighs probative value. Prior acquittals don’t automatically bar admission if evidence meets the standard. Appellate courts review admission for abuse of discretion, but improper admission of prejudicial 404(b) evidence often results in reversal given its powerful impact on panel deliberations.