7세기 중엽 고구려의 대외관계는 對唐政策을 중심으로 전개되었다. 고구려는 645년에 당의 침공을 격퇴함으로써 대내외에 자국의 군사력을 과시하고, 국제적 위상을 크게 높일 수 있었다. 다만 당이 곧바로 몽골초원에서 흥기하던 薛延陀를 격파하고, 羈縻體制를 확립함에 따라 고구려의 당군 격퇴 여파는 크게 확산되지 못했다. 태종 사망 이후 唐의 長孫無忌 정권이 온건한 대외정책을 펴자, 西突闕이 크게 흥기했다. 이에 고구려는 서돌궐을 통해 내륙아시아 방면의 세력과 연계를 도모하는 한편, 契丹과 신라에 대해 공세적인 군사작전을 전개했다. 고구려가 당의 온건책을 틈타 공세적인 외교정책과 군사작전을 전개하며 당을 견제할 기반을 확충한 것이다. 655년에 성립한 唐의 許敬宗 정권은 강경한 대외정책으로 선회하여 658년 서돌궐을 괴멸시켰다. 그런 다음 신라와 연합해 660년에 백제를 멸망시킨 다음, 곧바로 고구려 원정에 나섰다. 다만 당은 별다른 전과를 올리지 못했는데, 고구려 서북방에서 契丹과 鐵勒이 크게 흥기했기 때문이었다. 이때 고구려가 거란과 연계한 사실은 묘지명을 통해 확인되는데, 鐵勒과도 연계했을 가능성이 있다. 이처럼 고구려는 다양한 군사전략과 외교정책을 통해 당의 공격을 저지했다. 이에 당은 백제 부흥군과 鐵勒을 평정한 데 만족하며 664년 7월부터 泰山 封禪을 추진했다. 당이 고구려 원정을 보류한 것이다. 이에 고구려는 당과의 관계를 개선할 기회가 마련되었다고 판단하고, 태자 福男을 파견해 봉선의식에 참여했다. 현상적인 추이만 놓고 본다면, 고구려는 멸망 직전까지도 급변하는 국제정세에 비교적 효과적으로 대응했다고 평가할 수 있다. 그렇지만 이러한 평가로는 고구려가 내분의 와중에 갑자기 멸망한 원인을 찾기 어렵다. 당시 고구려가 급변하는 국제정세를 얼마나 정확하게 인식했는지, 또 새로운 국제질서에 대응하기 위한 보다 근본적인 대외정책을 수립하려고 고민했는지 등을 더욱 다각도로 검토할 필요가 있다.

In the mid-7th century, Goguryeo’s foreign strategies were centered around its relationship with Tang(唐). In 645, Goguryeo managed to repel Tang’s invasion, and presented itself as a formidable military presence in Northeast Asia. But right after that victory, Tang was able to crush the Seol Yeon-ta(薛延陀) force which had been steadily rising in the region of Mongolia. Tang also created a Gimi(羈縻) system in that region, so Goguryeo’s victory over Tang failed to generate lasting repercussions. After Tang Emperor Taizong’s death, the Changsun Wuji(長孫無忌) regime (of Tang) decided to proceed with a more moderate foreign policy, inadvertently giving room for the Western Turqs[西突闕] to gain power and momentum. Goguryeo wanted to exploit the situation by befriending some Inner Asian states through the Western Turq, while also launching aggressive military campaigns against Khitan and Silla. In other words, while Tang was maintaining a rather passive position, Goguryeo attempted to make both military and diplomatic headways to be able to counteract what Tang would have in mind for the future. Then, the Xu Jing-zong(許敬宗) regime of Tang -which was established in 655- decided to once again employ a more aggressive foreign policy, and made that policy more than evident when it destroyed the Western Turq in 658. Tang then allied itself with Silla and conquered Baekje in 660. It was at this point when Tang was ready to take on Goguryeo, but it was not successful in the beginning because the Khitan and Cheol’reuk(鐵勒) factions suddenly uprose in the Northwest side of Goguryeo. It is confirmed through certain tomb epitaphs that (in order to face Tang) Goguryeo allied itself with the Khitans, so it is also possible that Goguryeo reached a similar agreement with the Cheol’reuk as well. As we can see, Goguryeo was able to stop Tang and repel its attacks through military means as well as diplomatic methods. Tang had to pause its advances only after crushing the Baekje refugees trying to reclaim their homeland and putting down Cheol’reuk. And since July 664, the Tang government engaged in preparations for the TaiShan(泰山) mountain’s Bongseon(封禪) ceremony. Figuring that Tang was essentially putting the Goguryeo campaign on hold, Goguryeo decided to use the situation to enhance its relationship with Tang, and had its own Crown prince Boknam(福男) attend the ceremony himself. So, apparently Goguryeo was dealing with international conditions surrounding itself in a timely manner and even effectively. But in the end, it was not able to keep itself from being internally divided, or prevent its own ultimate fall. There must have been something that Goguryeo overlooked while trying to keep up with the rapidly changing international dynamics. It also seems Goguryeo lacked a comprehensive perspective to set a plan that would better ready itself to cope with them.