Silla

Silla is the Kingdom which united the three Kingdoms.
They had the strongest army, but it isn't that did not mean an absolute to win against to Goguryeo, and or even to Baekje. So they invited Tang Dynasty of China into the war.
But Tang had another goal, which was to occupy the some or all of the Kingdoms or some of them. After Silla won to against Baekje and Goguryeo, the armies of the Goguryeo, Baekje, and Silla had to fight against to Tang.
Today, there is a opinion that expresses ing the regrets about the decision of Silla that had another country involved into the unifying war.
Although about the half of the territory of Goguryeo became the area part of China, the three Kingdoms were unified, and the new age of Unified Silla began.

{That is interesting history}