India's Prime Minister Manmohan Singh recently described Maoist rebels as the biggest single internal security threat the country has ever faced.
In the first of three reports, the BBC's Jill McGivering spent three days travelling with Maoist fighters in the jungles of the central state of Chhattisgarh to find out why they have taken up arms.
We travelled by jeep in pitch darkness for about 10 hours through the jungle, led by a succession of different guides. Finally, we stopped and were led to a small clearing.
Most of the Maoist fighters are young, many in their teens
A flickering torch lit up the faces of about 30 or 40 young fighters, standing in line shoulder to shoulder, most dressed in military fatigues, all carrying guns.
Most of them seemed very young, certainly still in their teens. Some wore military boots, others wore flip-flops, some were simply barefoot.
Each one shook my hand, then raised their fist in a clenched salute with the words: Lal Salaam! Which means: Red Salute!
That night the monsoon rains broke. We were lying on the jungle floor on simple plastic groundsheets.
As the rain grew heavier, the Maoists hastily marched us to a nearby house, several one room mud-brick huts set round a mud yard. We lay on the floor of one of these with the chickens.
The next day started at 0500 with roll call. Then we watched about 20 members of the platoon as they were put through their paces by a commander - first exercise, then military drill.
They practised firing positions, standing, sitting and crouching and crawling on their stomachs through the undergrowth.
Their guns were a mixture. Most had wooden shafts and long single barrels. They had made some of them themselves in the camp, they said.
Others were the same old-fashioned Enfield rifles still used widely by the Indian police, taken perhaps during a raid on a police post.
Maoist militants, known here as Naxalites, have been fighting in India since the late 1960s. Until recently, they have basically been in pockets in the jungles in India's poorest states.
Every morning the fighters undergo rigorous training
More recently there has been more unity between these groups.
Analysts talk now about the emergence of the Red Corridor, a great swathe of Maoist militancy which stretches all the way from the border with Nepal, south through India to the sea.
Later I was introduced to a senior Maoist commander, Gopanna Markam, a veteran of 25 years with the guerrilla force. I asked him how he would describe what he was fighting for.
"We're fighting for a new democratic revolution in this country," he said.
"People are hungry, there's nothing to eat. They have no clothes. They have no jobs. We want development for the people. That's why people are coming to this fight."
I asked him about the Maoist groups in Nepal who may lay down their arms and join the democratic process.
At first he was cautious. The official response was still being determined by the leadership, he said, who would issue a statement in due course.
But events in Nepal were being watched closely, he said - adding that the changes there were giving them hope.
"We hope that revolution is successful," he said, "so that will be an example all over the world because there is a big propaganda that socialism is finished. But Nepal we're hoping will show the way."
The language used by the commander to talk about his ideology, and the Maoist ambition of capturing the country's cities and overthrowing the Indian state, was littered with Mao's phrases and terms, an echo from 40 years ago.
The Maoist insurgency in India began in the late 1960s
Attempts since then to discredit some of Mao's thinking or challenge his ideas were, the commander told me, just a result of revisionism. Mao's theory is universal, he said. The Chinese Revolution was a success.
Later, I sat with a more junior member of the platoon, a 26-year-old woman called Jaymati who joined the Maoists eight years ago. Her face was stern as she described why she had left her family for this nomadic life as a jungle guerrilla.
She had grown up seeing the fight for the poor going on around her, she told me, so she thought she should join it.
Even if she wanted change in her society, I asked, didn't she think there was a way of achieving that peacefully, without killing and violence?
She seemed impassive. "There's a lot of exploitation of poor people," she said, "and the only way is to make them powerful, to make them the ruler. People who create trouble for poor people, they should be dealt with. They should be finished. They should be eliminated."
Certainly violence in Chhattisgarh is steadily escalating. The number of people killed there, according to government figures, has doubled each year in the last few years. Most of the dead are civilians.