When the narcotics trial started in the high-paneled, dimly lit federal courtrooms here, the government hoped to demonstrate that the man who calls himself the world's biggest gambler is instead one of the nation's major drug smugglers.

Whether acknowledged Las Vegas high-roller Jamiel Alexander (Jimmy) Chagra is either or both remains to be decided by a federal jury that this week is expected to begin sifting through testimony that could send the 34-year-old Chagra to prison for life.

But whatever the outcome, this trial, which the prosecution refers to as the "Colombian connection," has opened a wide window on a world in which marijuana and cocaine are transported wholesale by sea, air and land from South America into the United States.

The proceddings, in which Chagra's principal accusers are admitted drug smugglers, also have provided an unusually candid glimpse of how federal authorities use the punishment of long prison sentences and the reward of lenient probation as an inducement to persuade drug dealers to cooperate.

There are those in Austin who think that both the government's and the defendant's reputations have suffered in the course of the trial, in which Chagra is accused of two counts of conspiring to smuggle narcotics into the country from Colombia, one count of possessing cocaine with the intent of selling it and an added charge of directing a "continuing criminal enterprise."

The government seems shakiest on this last and most serious charge, which carries a 10-year-to-life sentence and forfeiture of assets. Because of legal rulings, the prosecution was unable to get before the jury evidence that Chagra had smuggled drugs into Connecticut and Pennsylvania. And most of the testimony that makes out Chagra to be a "kingpin" of drug smuggling rests on the word of El Paso dirt farmer Henry Wallace, a convicted drug dealer whom the Chagra defense is depicting as the actual leader of the enterprise.

One major prosecution witness testified that Wallace, who is awaiting sentencing on two long-standing cocaine convictions, played at least as much of a role in the smuggling ring as Chagra did.

The 29-year-old Wallace, who acknowledges participation in 20 drug-smuggling episodes, said he decided to cooperate after he was arrested in New Orleans carrying $114,000 in cash. The money was confiscated but returned to him within a week after he agreed to testify.

It is understandable that neither Chagra nor Wallace would want to claim responsibility for the smuggling operation described in this trial.

Chagra is supposed to be a brilliant organizer, but the testimony has depicted him as a star-crossed bungler with a proclivity for buying trucks that would not run, boats that became lost and airplanes that either crashed in the Colombian jungle or developed engine failure and turned back.

Reviewing the testimony, the Austin American-Statesman concluded that Chagra's story would make a good plot for a book entitled "The Gang That Couldn't Smuggle Straight." During the trial two government witnesses testified that they had cheated Chagra of $70,000. And the ring was so inept at raising money that Wallace testified that he offered himself as human collateral to Colombian drug dealers in exchange for 13 pounds of cocaine worth $180,000.

As Wallace told it, the Colombians were unwilling to provide Chagra credit because of a series of past misfortunes that included the crash of a drug-laden DC6 aircraft in the jungle.

Indeed, the most difficult part of the government case may be in showing that Chagra's alleged drug business made any profits that he can be required to forfeit. The potentially most lucrative deal, the smuggling of 78 tons of Colombian marijuana on three converted freighters, ended in disaster Christmas week 1976 when the ships were intercepted off the Florida coast by the U.S. Coast Guard.

The marijuana, federal officials testified, went up in smoke in the Dade County (Miami) incinerator.

When the trial began, the government alluded to alleged ties between Chagra and "Mafia" figures in the East. No such ties have been shown, and it may be just as well.

"If the Mafia was mentioned in this proceeding, they'd be entitled to sue for defamation of character," said one knowledgeable drug figure here.

But interviews with narcotics agents, other drug figures and attorneys in Texas suggest that the smuggling ring described in the Chagra trial is far more typical than the smoothly run syndicate operations featured in television and movie fiction.

"There are just no Mr. Bigs," said one official. "There are a series of semi-bigs who sometimes have loose alliances with one another." One attorney describes the drug business in the Southwest as "a highly entrepreneurial one in which the risks are beginning to equal the profits."

If this is true, it may partly reflect a stepped-up crackdown both by federal drug officials and by Texas authorities. Increasingly, federal prosecutors are turning to the stiffer penalties embodied in the charges of "continuing criminal enterprise" or of "racketeering in interstate commerce" (RICO), a statute specifically devised to combat the moguls of syndicated crime.

Drug detection patrols have been increased along both sides of the 1,200-mile Texas-Mexico border, according to most accounts. The improved law enforcement and the continued spraying of the deadly insecticide Paraquat on Mexican marijuana crops have lessened both the appeal of smuggling and the value of the marijuana.

"Marijuana is a high-risk crop," says one admitted smuggler. "It is bulky, smelly, hard to store and - it goes bad. The Paraquat scare has just made a difficult business more difficult."

Because of the relative undesirability of Mexican marijuana and the difficulty of importing it, large-scale smugglers now prefer to bring in Colombian marijuana by boat to Texas or Florida ports. The next "continuing criminal enterprise" trial scheduled in Texas stems from the seizure of 44 tons of marijuana aboard the freighter Agnes Pauline off Beaumont in April 1978.

The other trend in southwestern drug smuggling is a preference for cocaine, which can be concealed easily, over marijuana. One Austin attorney who specializes in drug cases and used to represent many marijuana clients says that his business now consists almost entirely of representing persons charged with using or selling cocaine.

Drug officials in Texas, Nevada and California say that cocaine has become much more socially acceptable in the last few years and is now used in circles where it was once a rarity. In the Chagra case, witnesses talk almost casually of alleged use of the drug by the defendant and themselves.

After regionally renowned defense attorney Lee Chagra, Jimmy's brother, was murdered in an El Paso robbery last December, investigators made it known that the autopsy had detected cocaine in the deceased lawyer's blood. The prosecution, which surviving brothers Jimmy and Joseph Chagra, an associate defense counsel, have accused of a "vendetta" against the family, has made a special point in the current trial of putting testimony into the record showing that other Chagra family members allegedly used cocaine.

Though Lee is dead, his reputation as an effective antigovernment drug lawyer haunts the current proceedings, where one of the charges against Jimmy is the 1976 importation of 17,000 pounds of marijuana into Oklahoma. It is the prosecution's last chance to get a Chagra for this crime, on which Lee won acquittal for all of the 10 original defendants.

Another towering legal figure whose absence is noticed here is federal Judge John Wood, a stern "law-and-order" jurist who was scheduled to preside over the Chagra trial. Wood was shot to death outside his San Antonio home May 29. The killer has never been found.

But the prosecution has been well served by Judge William Sessions, who replaced Wood as the trial judge. Wood had sided openly with the prosecution in court and was reversed by appellate courts on various occasions. Sessions has been scrupulously evenhanded, making it more likely that any conviction will stand up on appeal.

Chagra, known to his associates in Florida as "Jim Alexander," and to Las Vegas gamblers at Caesars Palace as a gutsy high-roller who sometimes lost or won $1 million a week on craps or cards, has maintained a cool composure through most of the trial. He is the father of four daughters and he had been hoping, and predicting, that his fifth child would be a boy.

His wish was granted Thursday when his wife, Liz, gave birth to a son in Las Vegas. Asked in the corridor what he would name the boy, Chagra replied, "Junior," and then added with a sad smile, "alias Jim Alexander."