They are all based on Dynasty Warriors & Samurai Warriors by KOEI just have more of a strategy aspect to them over the Hack and Slash through giant mobs of enemies. Romance of the Three Kingdoms centers around the end of the Han Dynasty when Wei, Shu, and Wu fought each other over the control of China. The Wei was led by Cao Cao, the Shu by Liu Bei , and the Wu by Sun Quan. It's is mostly based of the novel Romance of the Three Kingdoms which was inspired by Records of the Three Kingdoms. Nobnaga's Ambition is similar where it's based on the Sengoku period in Japan with Nobunaga, Hideyoshi, and Tokugawa just in a Romance of the Three Kingdoms style instead of a purely historical view.