The alliance between Goguryeo and Wei broke down in 242, when King Dongcheon of Goguryeo moved into the area and plundered the Liaodong district of Xi'anping (西安平; near present-day Dandong, Liaoning) at the mouth of the Yalu River. Xi'anping was an important area that had been under Goguryeo control in the early 230s but was overrun by Wei forces during the Liaodong campaign in 238. It was crucial agricultural land that was home to the "Small River Maek" (小水貊) people, a branch of the Goguryeo people known for their excellent bows. In addition, Xi'anping provided Goguryeo with sea access, from where Goguryeo communicated with Eastern Wu in the past. Finally, a Goguryeo presence there would cut off the land routes between the Chinese heartland and the peninsular commanderies. The Wei court reacted most strongly to this apparent threat to their control of Lelang and Daifang.