Scrunched into the Himalayas between Nepal and Bhutan, Sikkim is about the size of Devon but with a greater surface area because so much of it is mountainous. The 22nd (and last) edition of Murray’s Handbook for Travellers to India claims, with sublime certainty, that ‘there are, in Sikkim, only 518 villages and 14,777 occupied houses’. Admittedly, that was in 1975. There were also 44 monasteries, 4,000 flowering-plant species, including 660 different sorts of orchids, and 6,700 kinds of butterflies and moths. It sounds like paradise, and it was. But in that same year, time was called on Sikkim’s existence as a sovereign kingdom. Indira Gandhi, desperate to deflect attention from the domestic challenges that led her to suspend India’s constitution in the so-called ‘Emergency’, lit upon the smallest of her country’s neighbours and sent in the army. Sikkim was ‘merged’ with India and its fairy-tale monarchy abolished, its unique biodiversity becoming just another part of India’s own inexhaustible variety.

Andrew Duff’s Sikkim is a richly researched and compellingly written account of the anomalies and intrigues that lay behind this cynical move. At one level the story is pure Disney. Amid the potted palms of Darjeeling’s Windamere Hotel, the shy crown prince, Thondup, a widower with two children, meets nineteen-year-old Hope, a starry-eyed American with long black hair and wearing a dirndl. Each is attracted by the other’s vulnerability. It is 1959 and the Dalai Lama has just fled neighbouring Tibet. They talk about ballet and Buddhism. She returns to college in New York and he goes back to Gangtok, Sikkim’s gingerbread capital, to deal with anti-monarchical dissent. Two years later they meet again at the Windamere. He asks her to dance, then to stay in his Himalayan Shangri-La. They fall in love and become engaged. Cue epiphytes and butterflies.

But the engagement is controversial. The princesses Coocoola and Kula, Thondup’s sensationally beautiful sisters, disapprove; so does the anti-monarchist Sikkim National Congress, led by the diminutive Kazi Dorji and his venomous Scottish wife, the self-styled ‘Kazini of Chakung’; and so too does Nehru’s India, suspicious of any international interest in its borderlands and at that time badly bruised following its 1962 war with China. The happy couple appear oblivious. They listen to Bach on the palace gramophone and introduce the twist to Gangtok.

The wedding, held in 1963, is the social event of the year. Gangtok has never seen anything like it. National Geographic runs a thirty-page feature and Time magazine reports ‘wedding parcels from Tiffany’s piled side by side with bundled gifts of rank-smelling tiger and leopard skins’. Grace Kelly’s nuptials a few years earlier spring to mind: Hope is ‘the first American girl to wed royalty since the daughter of a former Philadelphia bricklayer married Monaco’s Prince Rainier in 1956’. The Maharajah of Jaipur brings his own champagne. A Polish hairdresser from Calcutta is jeeped in to lacquer the ladies’ beehives, and John Kenneth Galbraith, representing the Kennedys, staggers around with an arthritic hip. The bride, appearing amid a fanfare from ‘10 ft-long Himalayan horns, braying conch shells, and booming bass drums’, wears a white ‘mokey’ and a dagger, the Kazini comes all in black and Coocoola steals the show in gold lamé. Before the year is out, the groom’s father dies. Prince Thondup is crowned Sikkim’s new chogyal, and Hope gives birth to a crown prince – at which point, they might all have lived happily ever after.

In reality, the marriage was a shaky one – Thondup drank, while Hope fumed about his girlfriends – and the kingdom was even shakier. The royal family represented the interests of the landowning Sikkimese, mostly either Bhutias from Tibet or indigenous Lepchas – both of them Buddhists. But the majority of the chogyal’s subjects were neither Buddhists nor landowners. They were overwhelmingly Hindu Nepalis, who had been migrating into Sikkim for over a century and who now looked to India to redress the blatant discrimination that denied them representation proportionate to their numbers in Sikkim’s lickspittle assembly.

India under Nehru and then Gandhi sympathised with their situation. There was no place for dynastic autocrats in their vision of democratic socialism. India’s own princely states had been hoodwinked into merging with the Indian republic and it was not entirely clear why Sikkim shouldn’t be absorbed as well. A treaty inherited from the British appeared to guarantee Sikkim’s sovereignty, but in practice its rulers had often been treated as Indian maharajahs and, unlike Nepal and Bhutan, the country had failed to get itself recognised by the United Nations. Its economy was entirely dependent on India, its defence and foreign relations had long been handled by India and, with its northern frontier closed following China’s occupation of Tibet in 1951, the only route into the country was via India. Dorji and his peroxide-blonde wife had a case. Their populist National Congress party was modelled on that of India and supported by it. Concessions from Gangtok, in the form of electoral reform, land redistribution and a purely constitutional role for the chogyal, were inevitable.

They were also urgent. In defence of its uncertain Himalayan frontiers, India had already fought a war with Pakistan over Kashmir and another with China over sectors of the Indo-Tibetan border. In 1965 a second war with Pakistan threatened to involve Sikkim itself. In support of Pakistan, the People’s Liberation Army advanced on Sikkim’s mountain passes, while Beijing issued an ultimatum for the withdrawal of the Indian troops stationed there. Only a massive Indian incursion across the Pakistani frontier in Punjab, followed by a speedily brokered ceasefire, pre-empted a Himalayan Armageddon and spared Sikkim the fate of Tibet. With the Indian frying pan looking a better bet than the Chinese fire, Thondup soldiered on, even while he asserted Sikkimese autonomy and his own sovereignty at every opportunity. Hope lobbied in New York and London. Thondup toyed with the idea of a new Indo-Sikkimese treaty. Gandhi bided her time.

Skilfully weaving all these strands – personal, national and geostrategic – into a pacey narrative, Andrew Duff hones in on the denouement of 1975. Like Sunanda Datta-Ray in his Smash and Grab: Annexation of Sikkim, he holds Indira Gandhi responsible for the final ‘merger’; but, nudged along by the discovery of a wealth of first-hand reminiscences, recently declassified documents and the WikiLeaks exposures, his research is wider and his conclusions more judicious. What an Indian political officer called ‘the last tango in Gangtok’ followed ineluctably from that fairy-tale romance.