In 584 the powerful country, Sui(隋) faced China unification. Goguryeo couldn't avoid the war against Sui(隋) in the near future.
If war broke out between Goguryeo and Sui(隋) it would be unambiguous that Silla would attack the south of Goguryeo. Goguryeo needed the partner to prevent Goguryeo from the attack of Silla. The international politics of Goguryeo began in earnest.
In 587 The envoy of Goguryeo were sent to Japan and the envoys of Japan were sent to Goguryeo. Goguryeo was seeking to counter balance Silla through Japan. Japan posted a massive concentration of 20,000 soldiers on Kyusu(九州) for four years from 591 to 595.
Silla was afraid of the invasion of Japan and started to construct a fortress. Three fortresses were built in the capital of Silla from 592 to 593. The main-force units were sent to the eastern coast to prevent the attack of Japan.
When Silla puta massive concentration of soldiers on the south line Goguryeo invaded and occupied the south of Silla in the early 590.
