The office of Paegangjin was established for managing the area of Paegangjin
by Silla. Recently, the author has discovered the fact that Silla established the
guarding frontier in the western costal area of Hwanghaedo and the southern area
of Daedong river, with Balhae’s attack on Dengzhou in 732. In this article, I
tried to clarify the location of Paegangjin and its jurisdiction, and the development
process of the estuary of Yeseong river and inlands of Hwanghaedo which was
not dealt with in the other day. The result of the research shows that Paegaing-jin
can be regarded as Paegang-do/Paeseo-do which was the basis for the political
power groups of Koryeo. That is, Paegang-do was corresponded to Seohae-do,
ranging from the southern area of Pae river in Hwanghae-do towestern costal area,
in reign of King Seongjon in Koryeo. Paeseo-do can fall under the western area
of Pae river(Daedong river) under Seogyeong(Pyeongyang). And Gwannae-do
referred to Hanju in Silla territory.
In addition, Paegang-jin consisted of 14 Jins in and around Paegang-jin which
was described as a military region in Sindangseo(新唐書) but it was reduced to
12 Jins when it was reorganized in the King Heonduk. The development of the
northwestern border line in inland by Silla started when the border line was fixed
by establishing the guarding frontier in the area of Pae river and costal area of
Hwanghae-do, especially centering Janggu-jin. It is assumed that Silla organized
the ruling system on the area of Pae river through securing the estuary of Yeseong
river(748, establishing 14 Jin), the inland development around the area(762,
building 6 Seong) and lastly, organizing Gun-Hyun in the southern area of Pae
river(826, building Paegangseong).
The securing the estuary of Yeseong river and the development of inland
resulted in not only improving agriculture and commerce but also opening a
newport for trading with Tang. On the other hand, it was a strategy to protect
Silla itself from pirates and the influence of An Lushan Rebellion which emerged
as a new threat in northwestern border in the middle of 8th century.