Ancient times During the
Pleistocene, the Bohai Sea experienced numerous
glacioeustatic transgressions and
regressions, as evidenced by sediment cores sampled from the seafloor showing fluvial floodplain conditions during intervals of low sea level.
Imperial era Throughout imperial Chinese history, the Bohai Sea held both strategic and economic importance. During the
Tang Dynasty, it was a vital maritime link to the
Bohai Kingdom (698–926)—an early medieval Mohe-Korean state in what is now northeast China and parts of the Korean Peninsula. Trade and diplomatic missions between China, Korea, and Japan often passed through the Bohai Sea, making it part of the broader East Asian maritime network. During the
Liao (907–1125) and
Jin (1115–1234) dynasties, which were established by the Khitan and Jurchen peoples respectively, control over the Bohai region was central to their northern dominance. These dynasties maintained fleets and fortified ports along the coast. In the Yuan Dynasty (1271–1368), the Bohai Sea gained greater attention as maritime commerce expanded under a more unified empire. Naval bases were established along the coast, and shipping routes were actively developed to support both military logistics and trade. The Ming (1368–1644) and Qing (1644–1912) dynasties continued to use the Bohai region for maritime patrols and defense, especially as threats from
Japanese pirates (
wokou) increased during the Ming era. At the same time, regional ports such as
Yingkou and
Dagu became important for domestic salt, grain, and fish trade.
19th and 20th centuries Until the early 20th century, Bo Hai was often called the Gulf of Zhili () or Gulf of Beizhili (). The
romanization systems widely used in the West at the time rendered these names as variations of "Jili", "Chihli", "Pechihli", or "Pe-Chihli".
Zhili and
North Zhili were historic provinces in an area surrounding Beijing that approximately corresponds to what is now Hebei Province. In the last decade of 19th century, Bohai Gulf became the major stage of military conflicts. In 1894-95, the
First Sino-Japanese War broke out, and it featured critical naval engagements in and around the Bohai Sea. One of the most decisive battles was the
Battle of the Yalu River in 1894, fought just outside the Bohai Sea's eastern approaches, where the Qing navy's
Beiyang Fleet suffered a major defeat. This loss opened the path for Japanese forces to project power deeper into the Bohai region. Subsequently, in early 1895, Japanese troops invaded the Shandong Peninsula and captured
Weihaiwei, a heavily fortified naval base at the eastern mouth of the Bohai Sea. The fall of Weihaiwei marked the collapse of Qing maritime defense and helped force China to sign the humiliating Treaty of Shimonoseki, in which it ceded Taiwan and recognized Korea's independence. Several years later, during the
Boxer Rebellion (1899–1901), the Bohai Sea again became strategically important. As the anti-foreign Boxer uprising spread across northern China and reached Beijing, the port city of Tianjin, located at the western edge of the Bohai Sea, became a focal point for foreign military intervention. In June 1900, an international alliance of eight powers launched an amphibious landing at the
Dagu Forts, which guarded the mouth of the
Hai River and access to Tianjin and Beijing. After a fierce bombardment and ground assault, the Dagu Forts fell to the alliance forces. This opened the way for allied troops to advance inland through Tianjin and eventually reach and occupy Beijing, forcing the Qing court to flee and leading to the signing of the
Boxer Protocol in 1901. == Geography ==