After months of fitful starts and maddening delays, the international community is gearing up to deal with one of the great human tragedies of the 20th century -- the staggering catastrophe that is Cambodia.

At the United Nations tomorrow, foreign ministers and ambassadors of at least 33 nations will announce or discuss commitments to a relief effort that is finally under way despite enormous problems. Among the participants in this special Cambodia session will be Secretary of State Cyrus R. Vance, the foreign ministers of Australia, Canada, France and the Philippines, and lesser representatives of the world's most important nations, East and West.

On Capitol Hill, concerned lawmakers employed legislative sleight of hand last week to affix $30 million for Cambodian relief to measures already in the final stages of consideration. And in the Senate Friday, after hearing a chilling report by Sen. John C. Danforth (R-Mo.) of seeing Cambodians "literally dying before our eyes" on a recent official mission to Phnom Penh, the authorization was doubled, to $60 million, with unanimous whoops of arrival.

Two barges carrying 3,500 tons of rice purchased by private organizations, OXFAM and the American Quakers, are chugging through the Gulf of Siam toward a landing this afternoon in the Cambodian port of Kampong Som, formerly known as Sihanoukville. A UNICEF-International Red Cross daily C130 transport flight from Bangkok, with 15 tons of food and medicine, will be landing at Phnom Penh airport, augmented today by a special flight from Europe of freight-handling equipment and school supplies.

The international effort still is hobbled by political and logistical difficulties and falls far short of meeting even the minimum requirements of a people beset by famine, malnutrition and disease.

The Vietnamese-backed regime in Phnom Penh has rejected a U.S. proposal to truck in food from Thailand in large quantity, apparently because this could interfere with military operations against Chinese-backed insurgents in western Cambodia.

The single United Nations-sponsored food flight daily from Bangkok must fly around the southern rim of Cambodia and into Phnom Penh through Vietnam, more than doubling the distance and the fuel requirement and reducing its food load by half, because it is not permitted to fly over the western Cambodia battle zone.

About 10,000 tons of internationally sponsored food and relief supplies received so far by ship, and lesser tonnages received by air, seem impressive until they are compared with a bare subsistence goal, calculated by the International Red Cross. The Red Cross estimates about 1,000 tons of food every day are needed just to provide an emergency diet to the 2.5 million Cambodians reported to be in need.

Dr. Jean Mayer, the internationally known nutrition expert, has estimated that a diet adequate to strengthen the weakened Cambodians would require food support of 2,000 to 2,500 tons a day, twice as much as the international goal.

The enormity of the problem, as well as its tangled history and current complexity, is a barrier to public understanding and international action. U.N. Secretary General Kurt Waldheim has called it "a national tragedy, the proportion of which may have no parallel in history." President Carter has called it potentially "a tragedy of genocidal proportions." Many have compared Cambodia's plight to the Nazi extermination of Jews in World War II.

The numbers of those who have died from war, execution, forced uprooting, invasion, famine and disease will never be known accurately, but there is common agreement that the events of the past decade have reduced Cambodia's population from close to 7 million to less than 5 million. (Some estimates are of an even greater toll.) Thus a third of a nation has vanished, and at least half of those remaining are threatened with starvation.

In the opinion of several Asian experts, it is not hyperbole but simple fact that this ancient people is threatened with extinction as a functioning society. And like a critically ill patient, Cambodia is afflicted with an interacting set of crippling conditions that make it nearly impossible to distinguish one affliction from another.

The Khmer (Cambodia) nation is a very old one that flourished in the temples of its capitol at Angkor a thousand years ago and once held sway over most of present day Indochina. Under attack from its more adaptive and fiercer neighbors, the Thais and the Vietnamese, the Khmer empire shrank and stumbled. For nearly 500 years Cambodians have had to struggle to maintain themselves against encroachment from outside.

The initial phase of the current tragedy began with the secret U.S. bombing of the eastern Cambodia jungle redoubts in early 1969 and the overthrow of neutralist Prince Norodom Sihanouk by a U.S.-oriented group in 1970. Cambodia increasingly was wracked by war until the U.S. pullout from Phnom Penh and the triumph of the communist Khmer Rouge headed by Pol Pot in April 1975.

The large-scale killing, "purification" from foreign influences, forced depopulation of the cities. Attendant deaths from exhaustion and disease took a vast, uncounted toll. The Vietnamese invasion that began last Dec. 25 and the resulting military occupation, guerrilla war and starvation added to the tragedy.

In this proxy war of the communist great powers, Moscow is supporting the Vietnamese occupation and administration of the country, which is conducted in the name of a small group of Cambodians headed by Heng Samrin. Peking is supporting the resistance struggle waged by the remnants of the Khmer Rouge army headed by Pol Pot.

Starting early this year, the United States began receiving reports of serious food shortages which, together with uncertainty that the rice crop was being planted, suggested a famine ahead. American diplomats in Bangkok and some in Washington believed the reports in their darkest interpretation, but the Central Intelligence Agency and State Department intelligence analysts did not.

A CIA-State Department intelligence analysis in June reported that, despite some severe localized shortages, Cambodia would eke through without a widespread famine. Ambassador Morton Abramowitz in Thailand, where a growing number of emaciated Cambodian refugees gave first-hand reports, strongly objected that the intelligence was to optimistic.

At the urging of his East Asian bureau, Secretary of State Vance asked for intelligence satellite coverage of Cambodia to settle the argument. Because of heavy clouds over the flat, streamy land, it was not until early August that a photographic look at 7 percent of the country's arable land was possible. The result: the jarring estimate that only 10 percent of Cambodia's normal crop of rice, its staff of life, had been planted.

The United States earlier had taken steps to warn of possible famine and to urge international efforts to deal with it. Now the diplomatic activity was stepped up. On Aug. 9, immediately after receiving the new intelligence, the State Department announced the United State's "deep concern over growing evidence of famine" and urged maximum humanitarian assistance.

The United States made private presentations in August to more than 30 nations, including the Soviet Union, Vietnam and China, urging that they use their influence to bring about a speedy program of relief to Cambodia.

The communist governments reacted with suspicion and irritation to the U.S. overtures. The Soviets and Vietnamese first denied there was any major problem, then accused the United States of being in league with the Chinese in a plot to strengthen the insurgents. The Chinese expressed fear that the naive Americans would strengthen the shaky Phnom Penh regime by placing food in its clutches.

Food has been a weapon, in some respects the most important weapon, in this fierce Asian struggle for power and influence. The Vietnamese forces in particular are seeking to starve their enemies -- as the United States did to its Vietnamese foes through food denial tactics a decade ago.

Nevertheless, the Soviet Union and its East European allies sent 200,000 tons of food and supplies, an estimate from Moscow that is accepted by State Department officials. But this aid was strictly controlled, with political guidelines well in mind, and U.S. experts assume that large amounts were consumed by the Vietnamese military.

Under laws passed by Congress to thwart any Carter administration tendency toward reconciliation on generous terms, U.S. aid to Cambodia specifically was forbidden. Emboldened by a legal loophole and growing congressional concern about starvation, the State Department authorized aid to feeding programs for Cambodian refugees in Thailand, which also sent food across the border, and later approved relief aid to Phnom Penh through international agencies.

Jacques Beaumont of UNICEF and Francois Bugnion of the International Red Cross, who were to negotiate with the Phnom Penh government on behalf of the international community, arrived in the Cambodian capital Aug. 9 with the outlines of a relief plan. (The two had made a brief introductory trip in mid-July).

They found a shattered city, a skeletal government, a devastated populace and deep suspicion of the motives of outsiders.

The ministries charged with arranging and distributing food and supplies were starting anew without staff, policies, experience, communications, files or even office supplies. Beaumont, who has spent 59 days of the past three months negotiating in Phnom Penh, recalls officials picking up waste paper from the floor to use for notes or directives.

"Things which are normal and expected in our world had simply disappeared. They had to face a situation in which everything had to begin again, at the same time and from scratch," recalled Beaumont in a telephone interview last week.

The Phonm Penh regime, with Vietnamese backing, strongly resisted United Nations insistence that food must go to all in the country, including civilians in insurgent-held areas. Phnom Penh never changed its position, but it slowly acquiesced to an expansion of international relief efforts that respect the principle.

It was Sept. 23 before Phnom Penh agreed to an expansion of UNICEF-Red Cross staff in the country, an act which was interpreted by the agencies as a green light for relief operations. Daily C130 flights from Bangkok began Oct. 13. Meanwhile, many thousands were dying and hundreds of thousands growing weaker.

The gravest problem is not obtaining food or medical supplies, but distributing it within the war-torn country. The ports and airfields are in poor condition, lacking basic equipment. Routes 5 and 6, the main arteries, are believed passable, but trucks, drivers, mechanics and tools are extremely scarce.

Ten food distribution trucks, the nucleus of a fleet of 145 expected in Cambodia by the end of this month, have arrived under U.N. auspices. But finding people to operate them will be difficult.

The U.S. "land bridge" proposal for hundreds of trucks operated from Thailand appears to be the quickest method to bring in and distribute large amounts of food. It is also the method that has generated the greatest suspicion and resistance from Phnom Penh. The fact that it was proposed by three U.S. senators in high-profile fashion has multiplied the suspicions.

The multifaceted catastrophe that has struck Cambodia is far from being solved or even alleviated. But in recent days, for the first time, it has captured the world's attention. That may be the first step on a long, long road back for a stricken people.