AN IMPORTANT conviction was obtained in a trial that ended last weekend in federal court here. Marcos Cadavid, a major figure in the Colombian drug trade, was found guilty of conspiracy to distribute cocaine in the Washington area between 1976 and 1983. In a series of transactions, he had been paid more than $20 million in cash.

The prosecution is significant as the first in what is expected to be a series of trials made possible by a new extradition treaty signed between the United States and Colombia. Until recently, the powerful and dangerous narcotics kingpins in Colombia were not only hard to prosecute in that country, they were also safe from prosecution for crimes committed here. But now, President Belisario Betancur's drive against drug dealers has produced dramatic results, in his own country and here.

Four Colombian traffickers have already been extradited and a number of sealed indictments have been handed up that will result in additional American prosecutions of Colombians. There has been some backlash from criminals in Colombia. The State Department, fearful of reprisals against Americans visiting that country, advised travelers to be particularly careful. But the crackdown continued. With the help of the armed forces, Colombian law enforcement officers moved in, seizing drugs, burning marijuana fields and making arrests. Some large producers have already moved their operations to neighboring countries.

This assault on the drug trade is important for its connection to crime in this country, and for what it means for the Colombian government. According to U.S. Ambassador Lewis Tambs, the growth of the narcotics trade has caused widespread addiction in Colombia and has handsomely financed armed revolutionaries operating in the rural regions.

Mr. Cadavid's six-day trial was conducted under tight security, for threats had been received that were taken very seriously. The jurors were sequestered; steps were taken to protect the judge and prosecutors; armed guards were even stationed on the roof of the courthouse when the defendant was moved to and from the building. It was all worthwhile. A major criminal will be sent to prison here for many years, and the Colombians have been reassured that American courts are a valuable ally in the effort to stem this vicious trade between the two countries.