The Samhan period of Korean history (also Proto–Three Kingdoms Period) comprises confederacies of Mahan, Jinhan, and Byeonhan in central and southern Korean peninsula, during the final century BCE and the early centuries CE. These confederacies were eventually absorbed into two of the Three Kingdoms of Korea by the 4th century CE. The Samhan period is generally considered a subdivision of the Three Kingdoms Period.
Sam (三) means "three", and Han is a Korean word meaning "great" or "leader" (cognate with "khan" used in inner Asia for leaders). Han was transliterated into Chinese characters 韓, 幹, or 刊, but is unrelated with the Han in Han Chinese and the Chinese kingdoms and dynasties also called Han (漢, 韓). Ma means south, Byeon means shining and Jin means east. The names of these confederacies are reflected in the current name of South Korea, Daehan Minguk (literally, "Great Han People's Nation"). See Names of Korea.
The Samhan are thought to have formed around the time of the fall of Gojoseon in northern Korea in 108 BC, around when the state of Jin in southern Korea also disappears from written records. By the 4th century, Mahan was fully absorbed into the Baekje kingdom, Jinhan into the Silla kingdom, and Byeonhan into the Gaya confederacy, which was later annexed by Silla.
Three Hans 
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The Samhan are generally considered loose confederations of walled-town states. Each appears to have had a ruling elite, whose power was a mix of politics and shamanism. Although each state appears to have had its own ruler, there is no evidence of systematic succession.
The name of the poorly understood Jin state continued to be used in the name of the Jinhan confederacy and in the name "Byeonjin," an alternate term for Byeonhan. In addition, for some time the leader of Mahan continued to call himself the King of Jin, asserting nominal overlordship over all of the Samhan confederations.
Mahan was the largest and earliest developed of the three confederacies. It consisted of 54 minor statelets, one of which conquered or absorbed the others and became the center of the Baekje Kingdom. Mahan is usually considered to have been located in the southwest of the Korean peninsula, covering Jeolla, Chungcheong, and portions of Gyeonggi.
Jinhan consisted of 12 statelets, one of which conquered or absorbed the others and became the center of the Silla Kingdom. It is usually considered to have been located to the east of the Nakdong River valley.
Byeonhan consisted of 12 statelets, which later gave rise to the Gaya confederacy, subsequently annexed by Silla. It is usually considered to have been located in the south and west of the Nakdong River valley.
The exact locations occupied by the different Samhan confederations are disputed. It is also quite likely that their boundaries changed over time. Samguk Sagi indicates that Mahan was located in the northern region later occupied by Goguryeo, Jinhan in the region later occupied by Silla, and Byeonhan in the southwestern region later occupied by Baekje. However, the earlier Chinese San guo zhi places Mahan in the southwest, Jinhan in the southeast, and Byeonhan between them.
Villages were usually constructed deep in high mountain valleys, where they were relatively secure from attack. Mountain fortresses were also often constructed as places of refuge during war. The minor states which made up the federations are usually considered to have covered about as much land as a modern-day myeon, or township.
Based on historical and archeological records, river and sea routes appear to have been the primary means of long-distance transportation and trade (Yi, 2001, p. 246). It is thus not surprising that Jinhan and Byeonhan, with their coastal and river locations, became particularly prominent in international trade during this time.
The Samhan saw the systematic introduction of iron into the southern Korean peninsula. This was taken up with particular intensity by the Byeonhan states of the Nakdong River valley, which manufactured and exported iron armor and weapons throughout Northeast Asia.
The introduction of iron technology also facilitated growth in agriculture, as iron tools made the clearing and cultivation of land much easier. It appears that at this time the modern-day Jeolla area emerged as a center of rice production (Kim, 1974).
Until the rise of Goguryeo, the external relations of Samhan were largely limited to the Chinese commanderies located in the former territory of Gojoseon The longest standing of these, the Lelang commandery, appear to have maintained separate diplomatic relations with each individual state rather than with the heads of the confederacies as such.
In the beginning, the relationship was a political trading system in which "tribute" was exchanged for titles or prestige gifts. Official seals identified each tribal leader's authority to trade with the commandery. However, after the fall of the Kingdom of Wei in the 3rd century, San guo zhi reports that the Lelang commandery handed out official seals freely to local commoners, no longer symbolizing political authority (Yi, 2001, p. 245).
The Chinese commanderies also supplied luxury goods and consumed local products. Later Han dynasty coins and beads are found throughout the Korean peninsula. These were exchanged for local iron or raw silk. After the 2nd century CE, as Chinese influence waned, iron ingots came into use as currency for the trade based around Jinhan and Byeonhan.
Trade relations also existed with the emergent states of Japan at this time, most commonly involving the exchange of ornamental Japanese bronzeware for Korean iron. These trade relations shifted in the 3rd century, when the Yamatai federation of Kyūshū gained monopolistic control over Japanese trade with Byeonhan.
Goryeo historians identified Mahan, Jinhan, Byonhan with Goguryeo, Silla, Baekje respectively, which was supported by their works like Samguk Sagi, Samguk Yusa and Jewang Ungi. Thus, the term, Samhan was occasionally synonymous with Samkuk, the Three Kingdoms of Korea. That historical view was previously given by Choe Chiwon, a noted Confucian scholar and Historian in the late of the Silla. Apart from the geographical location of Mahan, the Chinese historical record History of Song defines the ethnical origin of the Jeong-an kingdom, a successor state of Balhae, as Mahan.
In the late Joseon period, that historical notion came under criticism by an early Silhak scholar, Han Baek-gyeom who emphasized the linkage between Mahan and Baekje in terms of the geographical location.
See also 
- Kim, J.-B. (1974). Characteristics of Mahan in ancient Korean society. Korea Journal 14(6), 4-10. 
- Lee, K.-b. (1984). A new history of Korea. Tr. by E.W. Wagner & E.J. Schulz, based on 1979 rev. ed. Seoul: Ilchogak. ISBN 89-337-0204-0
- Yi, H.-h. (2001). International trade system in East Asia from the first to the fourth century. Korea Journal 41(4), 239-268.