Bangkok, Thailand — When the monsoon mud in Kampuchea (Cambodia) dries up in a few weeks, it may do more than signal the start of another dry season of fighting.
It may also provide the first test for the new Coalition Government of Democratic Kampuchea against Vietnamese troops occupying the embattled nation.
The coalition, formed last June, brings together the communist Khmer Rouge with the two noncommunist factions of Son Sann and Prince Norodom Sihanouk.
The coalition is rather shaky: Each member retains its own armed forces and seeks out its own aid. The two noncommunist leaders made no effort to hide their mutual dislike during their visit to Washington last month.
Still, the coalition has considerable diplomatic support and won another United Nations General Assembly vote last month in keeping the Kampuchea seat in the UN.
Once the rain lifts in Kampuchea, the coalition will be faced with a more daunting challenge - proving that it is a military a well as a diplomatic power.
The main brunt of any Vietnamese attack will probably fall on the Khmer Rouge , which has the largest anti-Vietnamese force with an estimated 20,000 to 25,000 men. But the Vietnamese and their allies have threatened to give the other two coalition members ''a few kicks.''
The five neighboring nations of Thailand, Malaysia, Singapore, Indonesia, and the Philippines (ASEAN, or the Association of Southeast Asian Nations) and others who have provided aid to the noncommunist groups will be looking carefully to see if their aid was well spent. The Khmer People's National Liberation Front (KPNLF) led by Son Sann has been the main beneficiary of aid.
The United States' position on aid is more ambiguous.
After Prince Sihanouk and Son Sann talked with US Vice President George Bush and Secretary of State George Shultz last month in New York, a US State Department spokesman hinted broadly the US would be providing the noncommunist Khmers with nonmilitary assistance, but the aid does not yet seem to have arrived.
ASEAN officials clearly would like to see the US making a substantial - and well-publicized - contribution that would boost the coalition's standing and encourage some of Washington's allies to follow suit.
A major KPNLF problem seems to be food: Western diplomats quote KPNLF leaders as saying that 25 to 30 percent of all food aid gets ''sliced off in transit.''
The KPNLF's main military donors have been Singapore and China. Peking has provided some 6,000 rifles this year; Singapore, which made a generous gift of weapons at the beginning of the year, recently gave a further 3,000 rifles.
The other ASEAN members have been less forthcoming: Indonesia and the Philippines apparently have given nothing. Malaysia and Thailand are said by their allies to have expressed a willingness to help train the noncommunist groups, and Thailand is reported to have promised some communications and transport equipment.
Meanwhile, KPNLF leaders are debating whether to join the war against Viet troops in full force.
A prime mover in the front, Gen. Dien Dell, long ago let it be known he considers the Khmer Rouge the main long-term enemy. His dream is that the Khmer Rouge will grind down the Vietnamese at enormous cost to themselves, only to find themselves faced with a fresh, well-equipped KPNLF army. KPNLF forces are said to number 9,000 to 12,000 men.
For the moment his adversaries seem to have the upper hand. He resigned as commander in chief last month following the murder of a KPNLF regimental commander in an incident variously described as a conflict of interest in the smuggling business or the elimination of a ''rotten apple.''
The four-man committee that replaced him is said to favor small guerrilla operations against the Vietnamese. These might build up the KPNLF's reputation, but might also bring the wrath of the Vietnamese down on their heads.
Most Western observers who follow the KPNLF closely doubt they can withstand a Vietnamese attack.
Prince Sihanouk has been less able to benefit from Western largesse because he has no army or political organization. But he has the name that is the key to much of the coalition's support both inside and outside Kampuchea.
Some ASEAN officials are disturbed by Sihanouk's propensity for sending conciliatory signals to Hanoi. ''We don't know whether he's sending up trial balloons or is trying to make his own peace with Hanoi,'' an official complained. The Vietnamese are probably just as confused.