Why did China begin asserting wider territorial claims in the South China Sea during the 1970s?

China’s assertion of its territorial claims in the South China Sea intensified during the 1970s primarily due to the discovery of potential oil and gas reserves in the region.1

While China had maintained historical claims dating back to the 1947 "11-dash line" (later modified to a 9-dash line) established by the Nationalists, these disputes remained relatively dormant for decades. The shift toward active assertion in the 1970s was driven by the economic incentive of these newly identified natural resources. This period of increased activity included the 1974 seizure of the Paracel Islands from South Vietnam, marking a significant escalation in the regional conflict.1