Entering Sikkim from a neighboring Indian state is like traveling from one sovereign nation to another, with immigration police on both sides of the border stamping passports.

This unusually formal border crossing from one state to another within the same country shows just how hard it has been for New Delhi to integrate its isolated northeast region--including this formerly independent Himalayan kingdom taken over by India in 1975.

Sikkim, which juts up from the narrow "chicken neck" Siliguri corridor that connects the northeast to the rest of the country, is considered an oasis of calm in the midst of turmoil and violence that prevail in the other seven states and union territories of the region. It is a distinct region of India, nearly separated by Bangladesh and Bhutan, and bordered by Burma and China.

Currently, there are two full-scale armed insurrections in the region, including one in Manipur in which 21 Indian soldiers died in an ambush last month. A third state, Assam, has seen more than two years of sometimes violent agitation that cost the central government $1.25 billion to import oil before it was able to get the oil pipeline there turned back on. There is simmering unrest in the other four states that threatens to escalate.

The northeast is without doubt India's largest internal-security problem and raises fears in New Delhi that any signs of weakness by the central government could encourage other embryonic secessionist movements that threaten the fragile unity of this 35-year-old nation.

The situation is so unstable in this part of India that foreign correspondents are banned from visiting the northeast. Permits were issued for a trip here only because of the cremation rites for the deposed king of Sikkim, the former husband of American socialite Hope Cooke.

Despite the air of calm here, this Sikkimese capital city and the road to it through the Siliguri corridor are prime listening posts for events in the northeast.

The most troublesome state in the region is Manipur, where the banned People's Liberation Army appears able to attack with impunity despite the massive presence of the Indian Army and the capture by military units last July of its top leader, Bisheswar Singh.

An officer in the Army's Jammu and Kashmiri Rifles, Capt. Cyrus Pithawala, was awarded India's highest medal for gallantry in peacetime at Republic Day ceremonies in January for capturing Singh during an all-night fire fight that left Pithawala wounded.

Nonetheless, the PLA recovered enough from that loss to stage a well-planned ambush last month on a convoy of Army trucks in which 20 soldiers were killed and a dozen injured. According to Home Ministry officials, the insurgents mined a road, blew up the first vehicle in the convoy and opened up with automatic weapons on the soldiers.

No arrests have been made despite 16 months of intensive counterinsurgency activities in the state. More important, the PLA fighters got away with a machine gun, a Sten gun and five automatic rifles--all useful for future raids.

The PLA is just one of four insurgent groups operating in Manipur, and, according to Indian Army intelligence reports, there are signs that some of them may be cooperating with an underground insurgent group from neighboring Nagaland, the National Socialist Council. The 30-year-old insurgency in that state, just north of Manipur, appears to have quieted down.

In recent months there has been a rash of insurgent attacks on public buildings and police officials in Manipur--including raids on the All-India Radio station in Manipur's capital of Imphal and on the home of a deputy inspector of police on the same day. In September, eight top PLA leaders escaped from the Imphal jail.

"The rebels are not just a handful of misguided youth," wrote the Indian Express in a Feb. 22 editorial. "They appear to be in sizable strength, well-armed and determined, with a good intelligence network and considerable support, willing or under duress, from the ordinary people."

Minister of State for Home Affairs N.R. Laskar told the upper house of Parliament that some of the insurgents had received military training and communist indoctrination in Chinese-controlled Tibet--a common charge by officials of Prime Minister Indira Gandhi's government who frequently blame "a foreign hand" for India's troubles.

Another open revolt against government rule is under way in Mizoram, just south of Manipur, where fighters of the Mizoram National Front appear to have ignored efforts of its Delhi-based leader, Laldenga, to negotiate a compromise.

Fighting there has been going on for 16 years as the front's insurgents are able to retreat to an underground headquarters in the rugged, mountainous region where India, Bangladesh and Burma meet.

According to reliable sources here, the front has such a free hand in Mizoram that it actually collects taxes to purchase arms--most of which are bought from gunrunners in isolated areas of Burma.

As violence increased and the fighters ignored a cease-fire that Laldenga had worked out, the Gandhi government retaliated in January by breaking off talks and declaring the front an illegal group.

The government announced that 268 members have been arrested recently, but sources here said there are still at least 800 armed fighters hidden in the jungles. With the borders there generally undefined and porous, they are able to slip out of Mizoram to Burma or Bangladesh when the government moves against them.

Assam, a state that produces about one-third of India's domestic oil, remains in the throes of a two-year struggle led by militant students. The Gandhi government, moving carefully, was able a year ago to get the oil tap turned back on.

In 1980, the agitation in Assam was at its height and was considered India's most popular mass movement since Mahatma Gandhi's struggle to win independence from the British.

Despite two years of high-level talks, no agreement has been reached on the key issue of keeping "foreigners"--including Indians from the neighboring state of West Bengal--from Assam. The Gandhi government is working, however, to stop the influx of Bangladeshis seeking better farm land in Assam by increasing border security. The government agreed in January to erect a barbed-wire "Berlin Wall" along the border.

The various issues in this area, which is about the size of Colorado, are similar. They rest on ethnic and cultural differences between residents in this region and those in mainstream India, and fears of tribal peoples of the northeast that their separate identities will be submerged under the weight of people moving in from the rest of India.

There is also a strong feeling in this region that the northeast has been ignored. The government appears to be trying to redress any imbalance, and is providing more development funds.

The feeling of ethnic differences runs through this former kingdom, despite newly placed statues of Indian national heroes Mahatma Gandhi and Jawaharlal Nehru in main squares of this frontier-like capital. Many Sikkimese consider themselves separate from the rest of India.

Resentment lingers over what Sikkimese say was agitation stirred up by New Delhi and a manipulated 1975 referendum that gave it the legal basis to take over the kingdom. The Sikkimese, wrote respected Calcutta journalist Sunanda K. Datta-Ray, "were tricked and bullied into national suicide."

There are frequently expressed fears here that the influx since annexation of "plainsmen" from the densely populated Indian states of Bihar and West Bengal--estimated at more than 60,000 since 1975 on top of an existing population of 200,000--will swamp Sikkim's unique cultural heritage and ethnic identity.

This view is common among the Buddhist Lepcha and Bhutia tribesmen, who came here centuries ago, as well as the Hindu Nepalese, who followed much later but now make up two-thirds of the population. Both groups are hill people, and look and act differently from mainstream Indians.

Nonetheless, no political party here is pushing for a return to independence, and there appears to be no separatist movement in Sikkim as there is in much of the rest of the northeast.

"It's a very peaceful state. That's one of its virtues," said Gov. Homi J.H. Taleyarkhan.