Brief History on Sikkim
Not much is known about Sikkim's ancient history, except for the fact that the first inhabitants were the Lepchas or Rong (ravine folk). Lepchas are said to have come from to the region from the Assam and Myanmar side. During 1200 AD  Sikkim was absorbed by other clans from Tibet which  included the Namgyal clan, who arrived in the 1400's and steadily won political control over Sikkim. In 1642, Phuntsog Namgyal (1604-1670) became the Chogyal (king). He presided over a social system based on Tibetan Lamaistic Buddhism. His descendants of Phuntsog Namgyal ruled Sikkim for more than 330 years.

During the 1700's, Sikkim suffered continuous attacks from Nepal and Bhutan, after which it lost much of its territory. Nepalese also came to Sikkim and settled there as farmers. By the 1800's, Sikkim's population was culturally very complicated, and internal conflict resulted. In 1814-1815, Sikkim backed the British in a successful war against Nepal, and won back some of its territory, once lost. 

In 1835, the British East India Company acquired the health resort of Darjeeling from Sikkim. During the mid-1800's, Sikkim violently withstand attempts to bring it under British rule, but in 1861 it finally became a British colony. The British had access through Sikkim to Tibet, and Sikkim's independent status was recognized. 

In 1890, Britain and China signed a convention recognising the border between Sikkim and Tibet. Later, the British installed a political office to help the Chogyal of Sikkim run the internal and external functions of the kingdom. 

Sikkim had retained guarantees of independence from Britain when she became independent, and such guarantees were transferred to the Indian government when it gained independence in 1947. A popular vote for Sikkim to join the Indian Union failed and Indian Prime Minister Jawaharlal Nehru agreed to a special protectorate status for Sikkim. Sikkim was to be a tributary of India, in which India controlled its external defence, diplomacy and communication. A state council was established in 1955 to allow for constitutional government for the Chogyal, which was sustained until 1973.
In 1962, India and the People's Republic of China went to war. Although Sikkim was an independent country, skirmishes occurred at the Nathula Pass between Indian border guards and the Chinese soldiers. After the war, the ancient pass was shut down (it reopened July 6, 2006).

Chogyal Tashi Namgyal succumbed to cancer in 1963 . Chogyal Palden Thondup Namgyal, ascended the throne in 1965. Trouble began to brew for the crown even before the Chogyal assumed the throne, as Indian Prime Minister Nehru, who had carefully preserved Sikkim's status as an independent protectorate, died in 1964. The chogyal, who responded to the increased pressure by drinking, was viewed by India as politically dangerous, especially after his wife, an American socialite Hope Cooke, published a journal article advocating a return of certain former Sikkimese properties.
In early 1970 the anti-monarchy Sikkim National Congress Party demanded fresh elections backed by greater representation of the Nepalese settlers.

In 1973, anti-royalty riots in front of the palace led to a formal request for protection from India. India worried that an unstable Sikkim would invite Chinese to act on its claims that Sikkim was part of Tibet, and therefore part of China. The Indian government appointed a Chief administrator, Mr. B. S. Das, who effectively wrested the control of the country away from the Chogyal.

Frosty relations between the Chogyal and the elected Kazi (Prime Minister) Lhendup Dorji resulted in an attempt to block the meeting of the legislature. The Kazi was elected by the Council of Ministers which was unanimous in its opposition to the retention of the Monarchy. Matters came to a head in 1975 when the Kazi appealed to the Indian Parliament for representation and change of status to statehood. On April 14, 1975, a referendum was held, in which Sikkim voted to merge with the union of India. Sikkim became the 22nd Indian State on April 26, 1975. On May 16, 1975, Sikkim officially became a state of the Indian Union and Lhendup Dorji became head of State (chief minister).

Chogyal of Sikkim

Phuntsog Namgyal (1604–1670) was the first chogyal (monarch) of Sikkim. He was consecrated in 1642 at the age of 38. Phuntsog was a fifth generation descendant of Guru Tashi, a 13th century prince from the Mi-nyak House in Kham in Eastern Tibet. According to legend, Guru Rinpoche, a 9th century Buddhist saint had foretold the event that a Phuntsog from the east would be the next chogyal of Sikkim. In 1642, three lamas, from the north, west, and south went in search for the chosen person. Near present day Gangtok, they found a man churning milk. He offered them some refreshments and gave them shelter. So impressed were they by his deeds that they realised that he was a chosen one and immediately crowned him king. The crowning took place Norbughang near Yuksom on a stone slab in a pine covered hill, and he was anointed by sprinkling water from a sacred urn.
Phuntsog, along with the lamas, then converted the local Lepcha tribes to Buddhism and set about expanding his kingdom up to the Chumbi Valley in Tibet, parts of modern day Darjeeling in the south, and parts of eastern Nepal.
Phuntsog moved his capital to Yuksam and instituted the first centralised administration. The kingdom was divided into twelve Dzongs, or districts under a Lepcha Dzongpon (governor) who headed a council of twelve ministers. During his reign Buddhism was consolidated as the established religion in Sikkim. He was succeeded by his son, Tensun Namgyal in 1670.
Tensung Namgyal (1644–1700) was the second chogyal (monarch) of Sikkim. He succeeded his father Phuntsog Namgyal in 1670. He moved the capital from Yuksom to Rabdentse near Geyzing in 1670. He had three wives and was succeeded by his son Chakdor Namgyal, borne by his second wife in 1700.
Chakdor Namgyal was the third Chogyal (king) of Sikkim. He was succeeded  Gyurmed Namgyal in 1717.
Gyurmed Namgyal was the fourth Chogyal (king) of Sikkim. He was succeeded by Phuntsog Namgyal II in 1733.
Phuntsog Namgyal II was the fifth Chogyal (king) of Sikkim. He was succeeded by Tenzing Namgyal in 1780.
During his reign the Nepalese raided Rabdentse, the then capital of Sikkim
Tenzing Namgyal was the sixth Chogyal (king) of Sikkim. He was succeeded by Tsugphud Namgyal in 1793. During his reign Chogyal fled to Tibet, and later died there in exile.
Tshudpud Namgyal (1785 - 1863) was king of Sikkim from 1793-1863. He gained independence from Nepal in 1815 and ruled under a British protectorate from 1861.
Under his father Tenzing Namgyal, most of Sikkim was appropriated by Nepal. Tshudpud Namgyal returned to Sikkim in 1793 to reclaim the throne. Because the capital of Rabdentse was too close to the Nepalese border, he shifted the capital to Tumlong.
Sikkim allied itself with the British in India, who also considered Nepal an enemy. Nepal overran most of the region, sparking the Gurkha War in 1814 with the British East India Company. The Sugauli Treaty and Treaty of Titalia returned the annexed territory to Sikkim in 1817.
In 1835, Tshudpud Namgyal ceded Darjeeling to the HEIC (Honourable East India Company) for an annual fee, but this relation was broken off sharply after he seized two British scientists in Sikkim, Joseph Dalton Hooker and Archibald Campbell. This led to two British military attacks in 1850 and 1861, resulting in the annexation of Sikkim by 1861. The same year, Tshudpud was granted the title of Maharaja of Sikkim by the British, and he abdicated the following year. At his death in 1863, aged 78, he had ruled Sikkim for 69 years, making him the longest-reigning Chogyal in history; as well, he was also the oldest ever Chogyal of Sikkim.
Sidkeong Namgyal (1819 - 1874) was king of Sikkim from 1863 to 1874. He was son of Tsugphud Namgyal and was succeeded by his half-brother Thutob Namgyal.
Thutob Namgyal (1860-11 February 1914) was the ruling Chogyal (monarch) of Sikkim between 1874 and 1914. Thutob ascended to the throne succeeding his half-brother Sidkeong Namgyal who died issueless. Differences between the Nepalese settlers and the indigenous population during his reign led to the direct intervention of the British, who were the de-facto rulers of the Himalayan nation. The British ruled in favour of the Nepalese much to the discontent of the chogyal, who then retreated to the Chumbi Valley and allied himself with the Tibetans.
After a series of skirmishes between the Tibetans and the British near Jelepla, the Tibetans were pushed back and the Chogyal was put under the supervision of Claude White, the appointed political officer in 1889. In 1894, he shifted the capital from Tumlong to the present location, Gangtok. He was knighted in 1911.
Thutob died in 1914 and was succeeded by his son, Sidkeong Tulku Namgyal. The Sir Thutob Namgyal Memorial (STNM) Hospital in Gangtok was built in memory of him in 1917.
Sidkeong Tulku Namgyal (1879-5 December 1914) was the ruling Maharaja and Chogyal of Sikkim for a brief period in 1914, from 10 February to 5 December. He was the eldest son and heir of Maharaja Sri Panch Sir Thutob Namgyal, and was educated at St. Paul's School, Darjeeling and at Pembroke College, Oxford. A polyglot, he was learned in Chinese, English, Hindi, Lepcha, Nepali and Tibetan. Following an attack of jaundice, Sidkeong Tulku Namgyal died of heart failure on 5 December 1914, aged 35. He was succeeded by his younger brother, Tashi Namgyal.
Tashi Namgyal (October 26 1893 – December 2 1963) was the ruling Chogyal (King) of Sikkim from 1914 to 1963. He was the son of Thutob Namgyal.
Namgyal was the 11th ruler of the Namgyal dynasty of Sikkim, succeeding his half brother Sidkeong Tulku Namgyal, who had ruled from February to December in 1914, till his death from heart failure. Born in Tibet and crowned by the 13th Dalai Lama, Thubten Gyatso, he was a strong advocate for closer links with India.
He was married in October 1918 to Kunzang Dechen, and they had 3 sons and 3 daughters. On his death he was succeeded as Chogyal by his son Palden Thondup Namgyal.
During his life, he favoured closer links between Sikkim, India and Tibet. Although some conspiracy theorists attribute his death to Indian agents, such theories are widely discounted by most historians due to his excellent relations with India.
About a decade after his death, his son Palden Thondup Namgyal, the incumbent hereditary Chogyal was formally deposed by the people of Sikkim who voted in a referendum (by a majority of 97%) to join the Indian Union. Palden Thondup Namgyal was widely unpopular among his people and the then democratically elected Prime Minister Lendup Dorji appealed to India to change the status of Sikkim from protectorate to statehood. On May 16, 1975, Sikkim was officially made the 22nd state of the Indian Union, thus ending the era of the Chogyal monarchy.
Palden Thondup Namgyal (May 23, 1923 - January 29, 1982) was the 12th and last Chogyal (king) of Sikkim.
At six, Chogyal Namgyal became a student at St. Joseph's Convent in Kalimpong, but had to terminate his studies due to attacks of malaria. From eight to eleven he studied in order to ordain monkhood under his uncle, Rimpoche Lhatsun, and was subsequently recognized as the reincarnate leader of both Phodong and Rumtek monasteries. He later continued his studies at St. Joseph's College in Darjeeling and finally graduated from Bishop Cotton School in Simla, in 1941.
Chogyal P.T Namgyal served as adviser for internal affairs for his father, Sir Tashi Namgyal, the 11th Chogyal, and led the negotiating team which established Sikkim's relationship to India after independence in 1949. He married Sangey Deki in 1950, a daughter of an important Tibetan family, and together they had two sons and a daughter. Sangey died in 1957.
Chogyal P.T Namgyal married Hope Cooke in 1963, a twenty-two-year-old American socialite who was a graduate of Sarah Lawrence College. The marriage brought worldwide media attention to Sikkim. The couple, who had two children, Prince Palden and Princes Hope Leezum, divorced in 1980.
Shortly after their marriage, his father died and Namgyal was crowned the new Chogyal on an astrologically favorable date in 1965. In 1975 Sikkim merged with the  union of India through a referendum receiving a 97%-majority election vote, thus ending his rule.
P.T. Namgyal was an amateur radio operator, call sign AC3PT, and was a highly sought contact on the airwaves. The international call book listed his address as: P.T. Namgyal, The Palace, Gangtok, Sikkim.
Chogyal Palden died of cancer in New York City, in the United States on January 29, 1982.He was cremated at Yumasala the royal cremation place near Hanuman Tok, People from all over Sikkim paid their respect and mourned his death.  
His son from his first marriage, Wangchuk Namgyal, was named the 13th Chogyal, but the position no longer confers any official authority.
Chogyal Wangchuk Tenzing Namgyal (b. April 1, 1953) is the second son of Palden Thondup Namgyal, the last sovereign king of Sikkim. He is also the present heir of the Namgyal dynasty and claimant to the Sikkim throne.
He was crowned as Sikkim's 13th king after his elder brother crown prince Tenzing Namgyal died in a tragic road accident. However, now this position does not confer any official authority and his responsibility includes only religious matters though he remains a chogyal in the hearts of Sikkimese people.