4 ideas
Sima Yi knew Zhuge Liang as a cautious man who never took risks, so he read the open gates as bait for an ambush and withdrew. The bluff worked only because the bluffer's long record of caution made reckless openness unbelievable.
A weak contender facing one dominant rival (Cao Cao in the north) and one entrenched rival (Sun Quan in the east) should not fight both. He should ally with the entrenched power against the dominant one and seize the neglected, defensible provinces of Jing and Yi as a base. Only after a stable three-way balance forms should he strike north from two directions when the dominant power falters.
After Wu killed Guan Yu, Liu Bei put his peach-garden oath of brotherhood above the Shu–Wu alliance that his whole strategy rested on. He invaded Wu and was destroyed by Lu Xun's fire attack at Yiling, which crippled Shu for good. The novel shows that the loyalty which drew followers to Liu Bei was the same trait that led him to strategic ruin when grief governed his decisions.
The novel opens by framing history as a cycle: an empire long divided must unite, and one long united must divide. Seen this way, no single hero's brilliance or virtue decides the outcome. Individual genius only rearranges players inside a structural swing, which is why none of the three founding houses wins and the Jin ultimately reunify the land.