In 1231, Goryeo found itself under the looming shadow of the Mongol Empire, which was sweeping across East Asia. The Mongol invasion of Goryeo was not merely a border dispute but the prelude to a forty-year long, arduous resistance that fundamentally shook the destiny of Goryeo. The immediate pretext for this tragic beginning was the assassination of the Mongol envoy, Jeogoyeo. In 1225, Jeogoyeo was killed by Goryeo people who resented the Mongols' excessive demands for tribute and their overbearing attitude, providing the Mongols with a perfect justification for invasion. However, underlying this was the expansionist ambition of the Mongol Empire, which had emerged as the hegemon of East Asia since the early 13th century, subjugating neighboring states. Goryeo, after its first contact with the Mongols in 1219, ostensibly to suppress Khitan remnants, had effectively fallen under Mongol influence and suffered from exorbitant tribute demands. In August 1231, a large Mongol army led by Salita crossed the Amnok River and invaded Goryeo. The Mongol forces advanced rapidly, reaching the vicinity of Gaegyeong (the capital), and numerous Goryeo fortresses either fell or surrendered. Recognizing the limitations of inland defense against Mongol cavalry tactics, the Goryeo court decided to relocate its capital to Ganghwado (Ganghwa Island), an island easily defensible by sea. In 1232, King Gojong and the court moved to Ganghwado, establishing a long-term resistance system. This was a strategic choice, exploiting the Mongol army's inexperience in naval operations, and it became the foundation for the forty-year resistance centered on Ganghwado. Unable to directly attack Ganghwado, the Mongols ravaged the mainland, engaging in plunder and massacre, but Goryeo persistently resisted, utilizing its geographical advantage as an island. The beginning of the Mongol invasion of Goryeo marked the start of one of the most severe and desperate periods in Goryeo history. While this forty-year resistance inflicted immense damage across Goryeo society, it also became a symbolic era demonstrating the resilient spirit and national pride of the Goryeo people. Great cultural legacies, such as the carving of the Tripitaka Koreana – the essence of Buddhist culture and an embodiment of the desire to overcome national hardship – were created during this period. Although Goryeo eventually succumbed to the Mongols and entered the Yuan intervention period, the relocation to Ganghwado and the long-term resistance were crucial factors that allowed Goryeo to maintain its independent dynasty and preserve its national identity, unlike other conquered nations. This was a decisive moment that allowed Goryeo to be recorded in history not merely as a subjugated people, but as a proactive force of resistance.