You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Yuan_dynasty

Context:
Instability troubled the early years of Kublai Khan's reign. Ogedei's grandson Kaidu refused to submit to Kublai and threatened the western frontier of Kublai's domain. The hostile but weakened Song dynasty remained an obstacle in the south. Kublai secured the northeast border in 1259 by installing the hostage prince Wonjong as the ruler of Korea, making it a Mongol tributary state. Kublai was also threatened by domestic unrest. Li Tan, the son-in-law of a powerful official, instigated a revolt against Mongol rule in 1262. After successfully suppressing the revolt, Kublai curbed the influence of the Han Chinese advisers in his court. He feared that his dependence on Chinese officials left him vulnerable to future revolts and defections to the Song.

Question:
Where did Korea border Kublai's territory?

northeast