From early 17th century, Tibetans (Bhutias) migrated to Sikkim. Bhutia Phuntsok Namgyal, became the ruler of Sikkim. He was a very capable administrator. During Phuntsok Namgyal's rule, Mahayana Buddhism became the state religion.
In the 18th century, Bhutan and Nepal captured some parts of Sikkim. Sikkim had to sign peace treaty with Nepal and Bhutan in 1775. The Gurkhas of Nepal and the Bhutias of Sikkim were always at war with each other.
In 19th century, British were successful in befriending Sikkim by making peace treaty between Nepal and Sikkim. In this period, Sikkim was forced to give Darjeeling to the British.
After India gained independance in 1947, the local parties of Sikkim wanted to join India. Still Tashi Namgyal managed to get a special status of protectorate for Sikkim.
By 1970, politicians of Sikkim demanded the removal of monarchy and establishment of a democratic setup. In 1973 the Sikkim Durbar collapsed completely. Indian Government appointed a Chief Administratorin the state. After the 1974 elections, Sikkim became an associate State, and Namgyal remained as the constitutional figure head monarch.After another confrontation between the Namgyal and the popular Government, Sikkim became a full-fledged 22nd state of the Indian Union on 16th May 1975. The institution of Namgyal was subsequently abolished.