Bangkok, Thailand — ''Striking at the source'' is what narcotics agents have long advocated as a most effective weapon against worldwide use of heroin.
Now a daring Thai military raid against a mysterious opium ''warlord'' has done just that.
Hardpressed narcotics agents throughout Southeast Asia, the United States, and Western Europe are pleased because the raid means this year's opium harvest in Southeast Asia's ''golden triangle'' may be considerably lower than expected. The result: lower heroin exports next year.
Still the agents maintain that unless Thai anti-narcotics raids are continued and expanded, the drop in heroin supply will be temporary.
In Western Europe ''golden triangle'' heroin has increasingly filled a vacuum left by Turkey's suppression of heroin exports. In the United States there is concern that still small but growing US imports of''golden triangle'' heroin (nearly 20 percent of imports) may gradually supplement the 75 percent of heroin imports coming from Pakistan, Afghanistan, and Iran.
The raids against Khun Sa, a half-Chinese military leader also known by his Chinese name as Chang Chi-fu, interrupted and in some places destroyed the harvest of opium, which is eventually converted into heroin.
But the long-term prospects for improvement also depend on expanded government assisted programs of ''crop substitution.'' Farmers in the region who have long supported themselves by cultivating poppies (from which opium is derived) must be taught that switching to other crops can also be profitable.
Khun Sa is considered responsible for 70 percent of Thailand 's heroin manufacture and distribution in the ''golden triangle''(where Thailand, Burma, and Laos meet).
Action against Khun Sa is also increasingly of interest to Southeast Asian countries. Narcotics takes a harsh toll on Southeast Asian society. Thailand has more than 500,000 addicts (of a population of almost 49 million). The police deal with almost 500 drug cases a week.
Malaysia has an equally grave problem. One in 30 of its people is addicted to some drug, in most cases heroin. In a two-week period last month, Malaysia's newly created narcotics division arrested 4,000 people. More than 100 are now facing death sentences or life imprisonment for drug trafficking.
Narcotics experts say last year's bumper crop lowered prices and raised consumption in the region.
But at least for the immediate future Khun Sa's losses of men, material, and territory during the latest raids will help curb the heroin trade. The loss of his headquarters and his communications and training centers is particularly damaging.
A Bangkok-based American narcotics official declares,''Informers have told us that heroin refineries on the Thai-Burma border had not been operating for three weeks because of military activities and a shortage of chemicals used to refine opium into heroin.''
Across the border in the Shan states of Burma, blight has reduced the yield of opium from poppies.
As things now look, overall production will still be large, with more than 50 tons in Thailand, 50 tons in Laos and a vast 500 tons in Burma. This will help offset losses in Khun Sa's production.
The Thai narcotics problem has another dimension. Opium is grown in mountainous regions that are cut off from the rest of the country during the six-month rainy season. Opium cultivation has become a way of life.
The poppy flower is the emblem of the region. Even the border patrolmen carry the blossoms in their rifle barrels. And opium is used as a common medicine for children. Some children grow up as addicts.
One farmer in the region explained that substitute crops such as coffee, kidney beans, and maize do not earn as much as opium. Last year he made $250 from coffee, $450 from beans, and $1,300 from opium.
Marketing substitute crops is a problem for Thailand and other countries, according to Chaiya Poonsiri, the governor of Chiang Mai province. Trade barriers in some countries keep substitute crop prices down, making trade unprofitable for Thailand.
In related crackdowns, more than 100 kilograms of heroin were seized by police in Southeast Asia and Australia in the first six weeks of 1982. Despite increased surveillance, police are not sure their success rate has improved. Still they hope that the following steps have improved their performance:
* International cooperation, particularly in the exchange of intelligence, is more organized.
* Police are better trained and better equipped.
* Law enforcement has in some cases improved when corrupt police officials who enriched themselves through the opium trade retired.
Thus there is special welcome for Thai Prime Minister Gen. Prem Tinsulanond's apparent new determination to wipe out Khun Sa's drug empire. The prime minister marked the recapture of Ban Hin Taek, where the warlord had his headquarters, by ordering a new name for the town. Princess Maha Chakri Sirindhorn, the king's daughter, chose ''Ban Therd Thai'' (''village uplifted to freedom'').