Yu Deuk-gong in his 18th-century work Balhaego, an investigation of Balhae, argued that Balhae should be included as part of Korean history, and that doing so would justify territorial claims on Manchuria.
Yu Deuk-gong hævdede i sit 18. århundrede arbejde Balhaego (En undersøgelse af Balhae), at Balhae burde indgå som en del af koreanske historie, og at dette ville retfærdiggøre territoriale krav på Manchuriet.