An independent Korean state or collection of states has existed almost continuously for several millennia. Between its initial unification in the 7th century - from three predecessor Korean states - until the 20th century, Korea existed as a single country. It became a Chinese tributary state in 1392 with the formation of the Choson Dynasty. Korea gained independence from China at the conclusion of the Sino-Japanese War in 1895 with the Treaty of Shimonoseki. Imperial Japan, following the Russo-Japanese War in 1905, forced Korea to sign the Protectorate Treaty beginning a period of Japanese occupation. In 1910, Tokyo formally annexed the Peninsula. Korea regained its independence following Japan's surrender to the United States in 1945. After World War II, a democratic-based government (Republic of Korea, ROK) was set up in the southern half of the Korean Peninsula while a Communist-style government was installed in the north (the DPRK). During the Korean War (1950-53), US troops and UN forces fought alongside soldiers from the ROK to defend South Korea from DPRK attacks supported by China and the Soviet Union. An armistice was signed in 1953, splitting the peninsula along a demilitarized zone at about the 38th parallel. Thereafter, South Korea under the PARK Chung-hee regime achieved rapid economic growth with per capita income rising to roughly 17 times the level of North Korea. In 1993, KIM Young-sam became South Korea's first civilian president following 32 years of military rule. South Korea today is a fully functioning modern democracy. President LEE Myung-bak has pursued a policy of global engagement since taking office in February 2008, highlighted by Seoul's hosting of the G-20 summit in November 2010 and its scheduled hosting of the Nuclear Security Summit in March 2012, as well as the 2018 Winter Olympic Games. Serious tensions with North Korea have punctuated inter-Korean relations in recent years, including the North's sinking of the South Korean warship Cheonan in March 2010 and its artillery attack on South Korean soldiers and citizens in November 2010.