Why did the Han people win more and lose less against foreigners before the Three Kingdoms, but were in a weak position after the Three Kingdoms?

Before the Tang dynasty, most monarchs advocated governing the country by force, and internal scuffles continued. As soon as the civil strife subsided, the soldiers became extremely elite under the baptism of war. After the Tang dynasty, they tended to be civilized, focusing on guarding the border, with less wars and relatively weak soldiers. At this time, foreigners are in a period of attacking each other. In fact, it does not mean that the Han people at that time were weak. It was just that the Han people at that time were used to a stable and peaceful life and had no desire to expand their territory. They just want to hold the existing territory, and then gradually become weaker and weaker, while the power of foreign countries is getting stronger and stronger through constant attacks.