The alliance was established between Silla and Tang dynasty to save Silla from nearby nations such as Baekjae or Goguryeo. Baekjae invaded Daeya Castle and expanded military operation to destroy Silla. During this attack, the son-in-law of Kim chunchu past away. A general of Kim chunchu went to Tang dynasty after he got refusal of Goguryeo to support Silla. The Silla and Tang associated together and made an alliance which is called "Silla-Tang alliance"
References
Silla-Tang alliance Wikipedia(Text) CC BY-SA