This was because Japan thought if the Korean Peninsula, which had acted as an intermediary for exchanges between Japan and the continent since ancient times, together with Ryukyu, was possessed by an adversarial country, it would catch hold of a strategically fatal weakness of Japan.
古来より永きに渡って琉球と並んで日本と大陸との交流におけるパイプ役を果たしてきた朝鮮半島が敵対国家に渡ることは、日本にとって戦略的に致命的な弱点を握られることを意味していると考えたためである。