The Koguryeo, Paekjae and Shilla kingdoms together formed the period known as the three Kingdoms era.
It lasted until the Shilla unified the three Kingdoms. The wars were fought with the Sui and Tang dynasties amongst the three Kingdoms. During this period, many weapons were developed, as well as, new equipment to protect soldiers from the newly developed weapons.
The understanding between the Kingdoms ended when Shilla pulled the Tang dynasty army on to the field. The Tang dynasty broke their treaty and tumed against the Shilla to acquire the lands of Koguryeo and Paekjae; the Shilla had to fight against the Tang dynasty to finish unification.