Sikkim became an Associate State of the Indian Union through

Sikkim became an 'Associate State' of the Indian Union through the Constitution (35th Amendment) Act, 1974 [2]. Following the lapse of British paramountcy in 1947, Sikkim initially became a protectorate of India, with the Indian government managing its defense, external affairs, and communications [1]. In 1974, the Sikkim Assembly expressed a desire for closer association with India, leading to the 35th Amendment [2]. This amendment introduced a unique category of statehood by inserting Article 2A and the 10th Schedule into the Constitution, specifically defining the terms of this association [1]. This status was short-lived; in 1975, the 36th Amendment Act was passed, which repealed the associate status, abolished Article 2A, and admitted Sikkim as the 22nd full-fledged state of the Indian Union [3].

Sources

1. [1] Indian Polity, M. Laxmikanth(7th ed.) > Chapter 6: Union and Its Territory > New States and Union Territories Created After 1956 > p. 55
2. [2] Introduction to the Constitution of India, D. D. Basu (26th ed.). > Chapter 6: TERRITORY OF THE UNION > TERRITORY OF THE UNION > p. 75
3. [3] Introduction to the Constitution of India, D. D. Basu (26th ed.). > Chapter 5: NATURE OF THE FEDERAL SYSTEM > NATURE OF THE FEDERAL SYSTEM > p. 64