Sikkim was originally populated by the Lepchas, a tribal people thought to have migrated from Assam in the 13th century. The Lepchas were essentially nature worshippers, moving from one forest clearing to the next as they spread throughout the valleys of Sikkim. With the migration of the Tibetans to Sikkim during the 17th century, the Lepchas were forced to move to the more remote regions of the country.
In 1641 the 5th Dalai Lama in Lhasa appointed Phuntsog Namgyal the first ruler of Sikkim. At this time the country included part of eastern Nepal, part of the Chumbi valley in Tibet, some of the western valleys of Bhutan and, to the south, Darjeeling , Kalimpong and the territory down to the Indian Plains.
Boundaries were altered as a result of the wars with Bhutan between 1717 and 1734. Sikkim lost much of the southern foothills, as well as Kalimpong on the important trade route between India and Tibet. More territory was lost after 1780, following the Gurkha expansion in Nepal. The boundaries were further realigned after the conflict between the British East India Company and the Gurkhas. The Treaty of Siliguri (1817) ensured that the territory lost to the Gurkhas was returned by the British assumed control over Sikkim's external affairs and its trade negotiations with Nepal, Tibet and Bhutan.
Further British expansion led to the declaration of Sikkim as a protectorate in 1861, and a further delineation of its orders the Tibetans, however, were becoming increasingly suspicious of British expansion and in particular regarded much of Northern Sikkim to reassert their authority the British resisted, and the powers of Sikkim's rajah were further reduced in 1890 a separate treaty declared Sikkim a semi-independent state, administered by the rajah under the guidance of a British Political Officer however, the Northern boundaries of Sikkim had never been clearly defined, and it was not until 1902 that a boundary commission was established to determine the current borders between Sikkim and Tibet
The British were keen to develop the country and encouraged workers from Nepal to settle in Sikkim it was a development which continued after 1947, when India became independent, and Sikkim became a protectorate within the Indian union indeed, in early 1960s the Nepalese constituted 75% of Sikkim's population during this time, Sikkim's Rajah (King) upheld a policy to prohibit further immigration and to restrict the rights of Nepalese to citizenship. Demonstrations followed, and the Rajah sought refuge in India. India intervened, the rajah abdicated in 1975 and Sikkim became the 22nd state of the Indian Union.