North Korea invaded South Korea on June 25, 1950. After receiving its endorsement from the United Nations, the United States landed its first troops in Korea on September 15. On October 1, the first anniversary of the founding of the People's Republic, UN forces crossed the 38th parallel into North Korea. There was some disagreement within China's leadership about how to react to the American push to the Chinese border. Mao and Zhou wanted direct military intervention, but most other Chinese leaders believed that China should not enter the war unless it was directly attacked. Lin Biao was Mao's first choice to lead the Chinese People's Volunteer Army (PVA) into Korea, but Lin refused, citing his bad health.