External threats faced by India, China, and Southeast Asia included foreign invasions and rivalry between kingdoms. Their responses involved building physical barriers like walls and forts, maintaining large armies, developing diplomatic relationships through tribute systems and alliances, and integrating foreigners by accepting them into their societies. China built the Great Wall and had large armies with advanced weapons like gunpowder. Kingdoms in Southeast Asia formed diplomatic ties but their technologies were sometimes useless against European invaders. Overall, the regions defended themselves through military and diplomatic strategies while also integrating outsiders over time.