Final answer:
The Song rulers were forced to move the imperial court south to Hangzhou due to the threat of invaders from the north.
Step-by-step explanation:
The Song rulers were forced to move the imperial court south to Hangzhou due to the threat of invaders from the north. In 1127, the Jin captured the Song capital of Kaifeng, and as a result, the Song emperor surrendered and the imperial family was marched off to the north. The southern Song dynasty was then established in Hangzhou, where they continued to develop the economy and strengthen their navy and army.