Cambodia: keystone of Hanoi's plans for Indochina. The onset of the dry season in Indochina, due soon, brings the most intensive military activity of the year. In the first of three articles, the Monitor looks at Cambodia under Vietnam: Hanoi's means of control, its objectives, and the impact of the war on Cambodians.

``B68,'' one of the most secretive organizations in Vietnam, oversees Vietnam's policy in Cambodia. It is said to have its headquarters on Tran Hung Dao street, the broad, congested avenue in Ho Chi Minh City that connects old Saigon with its twin city of Cho Lon.

``B'' stands for ban, which is Vietnamese for a commission. This one is attached to the Vietnam Communist Party Central Committee's foreign relations commission. What ``68'' stands for is unclear. The numbers are often dates, but this one is an enigma.

B68's operations in Cambodia (Kampuchea) are based on Hanoi's long-term economic and strategic framework for all of Indochina. Its apparent postwar objective is a comprehensive political, military, and economic alliance resembling a combination of the Soviet bloc military alliance, the Warsaw Pact, and its economic counterpart, the Council on Mutual Economic Assistance.

The political aspects of the alliance-to-be are already being handled by B68. Vietnam and Cambodia are bound militarily by a joint friendship treaty that was signed in February 1979 (a similar treaty exists for Laos). And the three nations' planning ministers have been meeting annually to discuss coordination of their five-year development plans.

Security is also a fundamental concern. The Vietnamese have decided that they cannot afford to have a neutral, potentially hostile neighbor on their western flank. Vietnamese statements on Indochina, regularly echoed by their Cambodian and Laotian allies, consistently drive home the message of Indochinese interdependence -- a threat to one country is a threat to all. The message is strikingly similar to the Brezhnev doctrine, advanced by the Soviets to justify their invasion of Czechoslovakia in 1968.

The head of B68, fittingly enough, is Le Duc Tho, one of the most powerful but most low-key members of the Communist Party leadership. Tho has been in charge of Vietnam's Cambodia policy since 1978. Before that, during much of Hanoi's war against the United States, he had responsibility for the war in southern Vietnam. Cambodia was an extension of that war for both sides.

In Cambodian capital of Phnom Penh, B68's extensive network is overseen by a team of deputy directors. One of these is Ngo Dien, currently also Hanoi's ambassador to Cambodia. Dien is an old Cambodia hand. In the 1960s he was the Phnom Penh correspondent of the official Vietnamese News Agency.

In those happier days of China-Vietnam relations, Dien used to teach French to Chinese diplomats; one of his former pupils is now one of Peking's liaison officers with the Khmer Rouge, the communist faction in the anti-Vietnam Coalition Government of Democratic Kampuchea. Dien went back to Cambodia for B68 in 1979, and among other things trained the first batch of Cambodian diplomats.

Dien watches foreign affairs for B68 and thus probably guides the formation of foreign policy by the Vietnam-backed government in Phnom Penh. He is said to be close to Phnom Penh's 34-year-old foreign minister and premier, Hun Sen.

Another B68 deputy director on second assignment from Hanoi's central committee bureaucracy handles the crucial issue of relations between the two ruling communist parties in Hanoi and Phnom Penh. A third, reportedly from the office of Hanoi's council of ministers, oversees economic policy. (Phnom Penh is showing first signs of adopting some of Vietnam's new economic policies.)

Security is handled by a vice-minister of the interior, Vien Chi. He controls the Vietnamese public security forces -- the Vietnamese interior ministry's own troops operating in Cambodia. Chi's troops reportedly arrested Phnom Penh's top leader, Pen Sovan in late 1981 when he fell from grace with Hanoi.

Military affairs are handled by Gen. Le Duc Anh, a southerner and Vietnamese Communist Party Politburo member. Anh is believed to spend much of his time in Phnom Penh and the northern province of Siem Reap.

B68 controls Vietnam's advisers to Phnom Penh. Some of these are purely technicians, filling in the war-created gaps in Phnom Penh's expertise. The most important but least-known advisers are probably those who work with the ruling Khmer People's Revolutionary Party (the communist party) in Phnom Penh. They give political guidance to the party and, presumably, early warning to the Vietnamese leadership of potentially dangerous deviations on the part of the Phnom Penh government. Cambodians are also trai ned in Hanoi and in a special political school in Thu Duc, outside Ho Chi Minh City.

In terms of actual military operations, Vietnamese military advisers train Phnom Penh's Army and accompany it in combat. The advisers are reportedly attached to Army units at all levels, down to the 35-man companies.

Vietnam's adversaries, notably the Americans and their allies in the Association of Southeast Asian Nations (ASEAN), claim that Vietnam is also trying to control Cambodia through the widespread settlement there of Vietnamese. But the Vietnamese actually moving into Cambodia are apparently doing so without permission.

While ASEAN is still waiting to see its prot'eg'e, the Kampuchean coalition (the two noncommunist factions led by Prince Sihanouk and Son Sann and the communist Khmer Rouge faction), get into its stride, the Vietnamese are already preparing for a tightly-allied postwar Indochinese community. Vietnam's abrupt replacement of Cambodian leader Pen Sovan in 1981 shows how far Hanoi will go to protect its interests.

This dogged commitment to security makes it highly unlikely that Hanoi will compromise with ASEAN on the key aspect of the Cambodia problem: political power.

Vietnam is willing to discuss the withdrawal of troops by 1990 because, as its officials say, their country's military presence in Cambodia is only a means to an end. They will even discuss the broadening of the Phnom Penh government to include noncommunists like Prince Sihanouk or Son Sann. But there is no sign that they will allow Phnom Penh's ruling communist party to lose its dominance of the government.

The energy that Hanoi is putting into Cambodia -- both in counter-insurgency and in building a long-term framework -- underlines the importance Hanoi attaches to consolidating Indochina. Few nations would perhaps have the confidence to work on a postwar framework before the war is over. But confidence has long been a trademark of Vietnamese communists. In 1974, after a particularly comprehensive crackdown against the communist underground in Saigon, an organizer was asked if he was dispirited.

``Oh no,'' he answered. ``You see, we've won already. Victory is inevitable. The other side just hasn't noticed yet.''