In September 732, the Balhae Dynasty attracted ‘pirates’ as allies, and had them raid on 登州, the main port of the Tang Empire, causing great damages. In addition, after the raid, these pirates carried out a commercial raiding strategy targeting the hostile countries along the Yellow Sea coast and sea routes in line with the strategic intention of the Balhae Dynasty. In other words, the Balhae Dynasty used these pirates like Privateers. This was a pattern that could be found universally in other eras and regions. It is presumed that the entity of these pirate groups was composed of local residents such as Goguryeo refugees or Mohe clans, who had been settled in a kind of warlords since the fall of the Goguryeo dynasty(668). Through various historical processes, a geopolitical buffer zone was formed from Manchuria to the northern part of the Korean Peninsula. That is why these pirate groups appeared. Above all, it should be taken into consideration that the situation was ahead of King Wu's expedition to 馬都山(March of the leap month in 733) according to the oath with Khitan and the Turks. In other words, it can be understood as an auxiliary attack in which irregular troops were mobilized for the purpose of supporting the main attack of the regular army of the Balhae Dynasty.
On the other hand, the activities of pirate groups were recognized as a serious threat to the Silla Dynasty. Therefore, immediately after the raid on 登州, the Tang Empire requested troops to attack the Balhae Dynasty, and the Silla Dynasty responded positively. Accordingly, the army of the Silla Dynasty attacked '靺鞨南鄙', through the northwest of the Korean Peninsula from the winter of 732 to the spring of 733. Furthermore, under the pretext of suppressing the '海曲之賊', the Silla Dynasty strongly demanded the Tang Empire to '浿江置戍', it was a Military and administrative authority for the management of the 浿江 basin, which had been neglected as a buffer zone between the two countries. As a result, it was finally approved in the spring of 735. Also, the Silla Dynasty used the title of ‘(持節) 寧海軍使’ to pressure or coax the regional groups, which had been the source of pirate activities.
‘浿江置戍’ had the meaning of finishing the Silla’s long-cherished desire for advancement to the north, since the reigns of Kings Muyeol and King Munmu in the past. On the one hand, in a practically, this was also to take a fundamental solution of the pirate problem that had been rampant along the coast of the Yellow Sea based on active intervention on the local region.
The ultimate result of ‘浿江置戍’ was the installation and operation of ‘長口鎭’. Through this, Silla was able to strengthen its control on the coast of the Yellow Sea and secure the safety of navigation. In other words, the pirate activities linked to the Balhae Dynasty in the 730s were a clear threat, but at the same time, they were also a major factor in the Silla Dynasty's advancement to the north.