After the fall of Busan, the Japanese army had to secure their bridgehead, because ten kilometres to the north of Busan lay the fortress of Dongnae, which was situated in a very strong position atop a hill,  dominating the main road north to Hanseong.
The fall of Busan led to the Japanese army needing to establish control over their defensive line.  Dongnae fortress, located strategically near the main road leading north towards Hanseong, was a key obstacle that needed to be secured.