It was under the control of the Russian Mafia until Colonel Peng offers the mercenary a contract to eliminate three Russian Mafia Capos, thereby pushing the Mafia out of the city and allowing the Chinese to occupy it.
Haeju is a port city located on the south-western portion of the Southern province, the city is comprised of an urban section where the citizens reside, which includes an orchard, and a small artificial lake with a gazebo suspended over the water. The port section of the city features warehouses, a refinery, and a train station. There is also a beach that goes along most of the west side of the city with a large statue of General Song located in the middle and many rocks lining the beach.
By the start of the game, Haeju is under the control of the Russian Mafia. Peng sends the mercenary to the city with the objective to eliminate three Capos, doing so will scare the Mafia into vacating the city, allowing the Chinese to move in, furthermore, the mercenary will receive a cash bonus if he completes the mission without ever being detected by the Mafia. One Capo is located at the beach, near the statue and rocks, guarded by an escort of several Thugs, which can make it hard to get him; another is located in the orchard with a few thugs and a Technical located around him and the final Capo is in the gazebo over the artificial lake, protected by a single thug.
A recommended approach would be to get hold of a chopper, preferably a small and quick one (or the MD-530 if you can gain control of one) and fly on top of a building. From there you can use a Sniper Rifle and pick off the capos and then quickly get back into the chopper and fly away.