Between 2008 and 2010, the anthropologist Alpa Shah spent 18 months as a participant observer in India’s largely rural state of Jharkhand. She lived among adivasis, tribal peoples outside the caste system who count among the communities most neglected by the government. Jharkhand is also one of the heartlands of India’s Maoist insurgency, a civil war that in 2006 the country’s prime minister identified as the “biggest internal security threat to the Indian state”. For decades, Indian politicians and commentators have argued about the country’s longstanding Maoist war: are insurgents ideological terrorists fixated on an outdated creed, or are they desperate rebels with a cause, forced to take up guns by state brutality? Dissatisfied by this polarised debate, Shah decided to immerse herself in the communities who live alongside the insurgents, to explore what the rebellion looks like from the grassroots.

Revolutionary insight … Alpa Shah. Photograph: Hurst Publishing

This was an exceptional undertaking. The geographic and cultural remoteness of these communities, together with the acute dangers of living in a warzone, mean few outsiders have based themselves there for longer than a few weeks. The lack of careful ethnographic investigation has permitted polemical views of the insurgency to dominate the Indian media. Even more remarkable is the fact that Shah spent her final week in Jharkhand’s forests disguised as a male guerrilla on a 150-mile (240km) trek with a Maoist platoon. Nightmarch – a report of her time with the Maoists and adivasi civilians they govern – provides one the most nuanced, informed accounts yet of this strange and awful conflict.

The civil war is in some ways a cold war anachronism. India’s contemporary Maoists trace their lineage to the Naxalite rebellion of the late 1960s, which was heavily influenced and encouraged by Mao’s Cultural Revolution. While that earlier conflagration was for the most part extinguished in the early 1970s by a harsh state response, splinters of the original movement fought on. In 2004, several of these fragments reunited within a new political and military organisation: the Communist Party of India (Maoist) and its People’s Liberation Guerrilla Army.

A Maoist conference in a forest, from Nightmarch. Photograph: Alpa Shah

The Indian government claims 20 of the country’s 28 states are affected. In reality, the Maoist operation is centred on central-eastern India: above all, on parts of Andhra Pradesh, Bihar, Chhattisgarh, Jharkhand and Odisha. It owes its survival to Maoist groups’ readiness to attack some of India’s socioeconomic enormities, such as the hierarchical violence of the caste system and racist exploitation suffered by adivasis. In the new millennium, the Maoists have gained further traction by linking their cause to environmental protests. After 2003, the Indian state – ambitious to increase taxation revenues – began granting lucrative mining contracts to multinational corporations, especially in mineral-rich Chhattisgarh and Jharkhand. Maoist insurgents organised locals into resisting state and corporate efforts to empty land ready for industrial development. A witness to state and corporation encroachment on tribal land rights, Shah describes “the juggernaut of perhaps one of the greatest people-clearing operations of our times”.

For some, joining the Naxalites is an adolescent rebellion, a way of escaping the control of their families, and experiencing the world beyond their village

Successive Indian governments have demonised and even criminalised any connection with or whiff of sympathy for the Maoist cause. In June this year, five human rights activists – defenders of civil society from state attacks – were arrested on charges of “Maoist links”. In August, at least five more were detained on the same pretext. Shah, by contrast, humanises the Maoists she meets. She evokes the self-sacrificing idealism of the movement’s leadership. Many senior Maoists were born into high-caste, educated clans, were swept up in global protest movements of the 1960s and 70s, then abandoned their families and elite career prospects to fight as full-time revolutionaries for some of India’s poorest people. Shah notes how Gyanji, the leader of the platoon with whom she marched, still retains the tender, light-skinned feet of his high-caste upbringing, 25 years after joining the Maoist “Jungle Sarkar” (forest state). He is in some ways an unlikely guerrilla, seemingly more interested in “the dance of starlings” and “European and Hindi-Urdu poetry” than in landmines.

Arriving at a Naxalite forest camp for a ‘cooking lesson’, from Nightmarch. Photograph: Alpa Shah

She is attentive also to the stories of rank-and-file adivasis, who join the Maoists for a bewildering variety of reasons. In the early 2000s, the Indian government sponsored the creation of local vigilante armies to fight Maoist control. Their scorched-earth destruction of villages accused of helping or harbouring Maoists drove many adivasis into the People’s Liberation Guerrilla Army. Others have smaller-scale grievances. A 16-year-old called Kohli ran away to the Maoists because his father slapped him for spilling a small cup of milk. For some, joining the Naxalites is an adolescent rebellion, a way of escaping the control of their families and experiencing the world beyond their village.

Yet Shah does not romanticise the Maoists or their relationship with adivasi communities. She bears witness to how, despite their stated idealism, their political dogmas glorify violence and foster corruption. The hardships of the adivasi existence (there is at best precarious access to food, medical care and education) notwithstanding, Shah is also sensitive to what it can teach those outside the jungle: for example, adivasi women enjoy far higher levels of gender equality than exists in caste-ridden Indian society. She worries that the Maoists’ contempt for tribal custom fundamentally erodes their claims to build popular democracy. “It is inevitable that their cultures will be obliterated with development,” one senior Maoist tells her.

Shah has only one long-term solution to the injustices of the continuing civil war: the proper exercise of India’s constitutional democracy, with full participation by those tribal communities long marginalised and even persecuted by it. Nightmarch – a considered, sympathetic and balanced analysis – is one of the few accounts we possess that gives them a voice.

• Nightmarch: Among India’s Revolutionary Guerrillas is published by Hurst. To order a copy (RRP £20) go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.