Modern Mongolia constitutes the area previously known as Outer Mongolia. The region was controlled for centuries by the Qing dynasty of China. The decline of said dynasty allowed Mongolia to declare independence in 1911. A monarchy under Bogd Khan (reigned 1911 - 1924), from 1919 to 1921 Mongolia served as a battleground for Republican Chinese forces, the White Russians (monarchists) and the Bolsheviks.
From 1921 to 1924, Bogd Khan had only limited powers while a Bolshevik-influenced government took major decisions. With his death the monarchy ended and the state was declared a People's Republic. It served mostly as a protectorate of the Soviet Union, receiving very limited international recognition. A Japanese invasion led to a brief Soviet-Japanese Border War (1939) with Mongolia as its battleground. The Japanese were decisively defeated and would never again attempt an offensive move against the Soviet Union.
In 1945, Mongolian forces joined the Red Army in an invasion of areas of Northern China controlled by the Japanese. The Soviets threatened to allow Mongolia to annex areas of Inner Mongolia, in case the Republic of China failed to recognize its independence. Chiang Kai-shek was forced to end the republic's territorial claims and recognize Mongolia. The People's Republic of China would follow in 1949.
Mongolia continued serving as a close ally and/or satellite state of the Soviet Union for the duration of the Cold War. A democratic revolution in 1990 forced the local Communist party to start reforms towards a multi-party political system. By 1996 they were completed and Mongolia had become a parliamentarian republic. Mongolia is the 19th largest state in the world but also the least densely populated. The capital and largest city is Ulan Bator. The state borders Russia and the People's Republic of China.