The Leafar Ambush took place in the year 0 ABY. During this event, Galactic Empire units launched a surprise attack on a Leafar merchant group. This happened at a crucial exchange location situated close to the Leafar system. To allow the Leafar to get away, a group of five Rebel X-wing starfighters intervened, engaging the Imperial forces. Additional Rebel backup was subsequently requested.
The Star Wars: X-Wing expansion, specifically Imperial Pursuit, depicts this incident in its Operation 16B (Support for Possible Allies).