The Republic of China (ROC) was established on 1 January 1912 as a sovereign state in mainland China following the 1911 Revolution, which overthrew the Manchu-led Qing dynasty and ended China's imperial history. From 1927, the Kuomintang (KMT) reunified the country and initially ruled it as a one-party state with Nanjing as the national capital. In 1949, the KMT-led government was defeated in the Chinese Civil War and lost control of the mainland to the Chinese Communist Party (CCP). The CCP established the People's Republic of China (PRC) while the ROC was forced to retreat to Taiwan; the ROC retains control over the Taiwan Area, and its political status remains disputed. The ROC is recorded as a founding member of both the League of Nations and the United Nations, and previously held a permanent seat on the United Nations Security Council until 1971, when the PRC took the seat of China from the ROC in the United Nations General Assembly Resolution 2758. It was also a member of the Universal Postal Union and the International Olympic Committee. The ROC claimed 11.4 million km (4.4 million sq mi) of territory, and its population of 541 million in 1949 made it the most populous country in the world.
The Republic of China was officially proclaimed on 1 January 1912 by revolutionaries under Sun Yat-sen, the ROC's founder and provisional president of the new republic, following the success of the 1911 Revolution. Puyi, the final Qing emperor, abdicated on 12 February 1912. Sun served briefly before handing the presidency to Yuan Shikai, the leader of the Beiyang Army. Yuan's Beiyang government quickly became authoritarian and exerted military power over the administration; in 1915, Yuan attempted to replace the Republic with his own imperial dynasty until popular unrest forced him to back down. When Yuan died in 1916, the country fragmented between local commanders of the Beiyang Army, beginning the Warlord Era defined by decentralized conflicts between rival cliques. At times, the most powerful of these cliques used their control of Beijing to assert claims to govern the entire Republic.