Korea is a peninsular region in East Asia consisting of the Korean Peninsula, Jeju Island, and smaller islands. Since the end of World War II in 1945, it has been politically divided at or near the 38th parallel between North Korea (Democratic People's Republic of Korea; DPRK) and South Korea (Republic of Korea; ROK). Both countries proclaimed independence in 1948, and the two countries fought the Korean War from 1950 to 1953. The region is bordered by China to the north and Russia to the northeast, across the Amnok (Yalu) and Duman (Tumen) rivers, and is separated from Japan to the southeast by the Korea Strait.

Known human habitation of the Korean peninsula dates to 40,000 BC. The kingdom of Gojoseon, which according to tradition was founded in 2333 BC, fell to the Han dynasty in 108 BC. It was followed by the Three Kingdoms period, in which Korea was divided into Goguryeo, Baekje, and Silla. In 668 AD, Silla conquered Baekje and Goguryeo with the aid of the Tang dynasty, forming Unified Silla; Balhae succeeded Goguryeo in the north. In the late 9th century, Unified Silla collapsed into three states, beginning the Later Three Kingdoms period. In 918, Goguryeo was resurrected as Goryeo, which achieved what has been called a "true national unification" by Korean historians, as it unified both the Later Three Kingdoms and the ruling class of Balhae after its fall. Goryeo, whose name developed into the modern exonym "Korea", was highly cultured and saw the invention of the first metal movable type. During the 13th century, Goryeo became a vassal state of the Mongol Empire. Goryeo overthrew Mongol rule before falling to a coup led by General Yi Seong-gye, who established the Joseon dynasty in 1392. The first 200 years of Joseon were marked by peace; the Korean alphabet was created and Confucianism became influential. This ended with Japanese and Qing invasions, which brought devastation to Joseon and led to Korean isolationism. After the invasions, an isolated Joseon experienced another nearly 200-year period of peace and prosperity, along with cultural and technological development. In the final years of the 19th century, Japan forced Joseon to open up and Joseon experienced turmoil such as the Kapsin Coup, Donghak Peasant Revolution, and the assassination of Empress Myeongseong. In 1895, Japan defeated China in the First Sino-Japanese War and China lost suzerainty over Korea and Korea was placed under further Japanese influence. In 1897, the centuries old Joseon was replaced by the Korean Empire with the Joseon's last king, Gojong, becoming the Emperor of the Korean Empire. Japan's further victory in the 1904–1905 Russo-Japanese War, expelled Russian influence in Korea and Manchuria. In 1905, the Korean Empire became a protectorate of the Empire of Japan. In 1910, the Empire of Japan officially annexed the Korean peninsula.

Korea under Japanese rule was marked by industrialization and modernization, economic exploitation, and brutal suppression of the Korean independence movement, as reflected in the 1919 March First Movement. The Japanese suppressed Korean culture, and during World War II forcefully mobilized millions of Koreans to support its war effort. In 1945, Japan surrendered to the Allies, and the Soviet Union and United States agreed to divide Korea into two military occupation zones divided by the 38th parallel, with the Soviet zone in the north and American zone in the south. The division was meant to be temporary, with plans for Korea to be reunited under a single government. In 1948, the DPRK and ROK were established with the backing of each power, and ongoing tensions led to the outbreak of the Korean War in 1950, which came to involve U.S.-led United Nations and communist Chinese forces. The war ended in stalemate in 1953, but without a peace treaty. A demilitarized zone was created between the countries, approximating the original partition.

This status contributes to the high tensions that divide the peninsula, and both states claim to be the sole legitimate government of Korea. South Korea is a regional power and a developed country, with its economy ranked as the world's fourteenth-largest by GDP (PPP). Its armed forces are one of the world's strongest militaries, with the world's second-largest standing army by military and paramilitary personnel. South Korea has been renowned for its globally influential pop culture, particularly in music (K-pop) and cinema, a phenomenon referred to as the Korean Wave. North Korea follows Songun, a "military first" policy which prioritizes the Korean People's Army in state affairs and resources. It possesses nuclear weapons, and is the country with the highest number of military personnel, with a total of 7.8 million active, reserve, and paramilitary personnel, or approximately 30% of its population. Its active duty army of 1.3 million soldiers is the fourth-largest in the world, consisting of 4.9% of its population. North Korea is widely considered to have the worst human rights record in the world.