First the U.S. Customs commissioner and an assistant secretary of state whacked the Mexicans for their inability to control their end of drug trafficking to the United States. Then a wrathful Mexican government whacked back. Whereupon the two administration officials were blind-sided by an unlikely authority on administration foreign policy, Attorney General Edwin Meese.

It was an unseemly slap-fest. But it was also an explosion waiting to happen. Perhaps both governments can now sit down in the spirit of thanks-I-needed-that and face the magnitude of the problem.

To the extent that the export of illegal drugs is countenanced as a consequence of official corruption, it has at least something in common with state-supported terrorism. It is just as lethal, in ways impossible to quantify (deaths by overdosing; drugged-driving accidents; drug-related crimes of violence). But you don't need to quantify precisely to know (a) that Mexico is our largest supplier of illegal drugs of all kinds and (b) that far more Americans die either directly or indirectly at the hands of illegal drugs than at the hands of international terrorism (a total of 23 terrorist-related deaths last year).

The difference, of course, is that drug trafficking would dry up without a market. "It's your problem," the Mexicans keep telling U.S. authorities. In theory, they're right. In practice, our own experience with Prohibition is a reminder of the difficulty of stamping out chemical dependency.

The point is not that the U.S. government couldn't do better. It's that the Mexican problem is overwhelming. A decade ago, the Mexicans seemed to be winning the war against illegal drugs. But the tide turned during the last five years when Mexico's oil bonanza collapsed, its economy crumbled and impoverished peasant farmers turned to the lucrative business of growing poppies and marijuana.

Almost overnight, illegal drugs have come to constitute what may be Mexico's biggest business, presided over by wealthy drug lords, aided and abetted by corrupt government officials. Result: drug-enforcement officials rate Mexico as the largest supplier of heroin to the United States, a close second to Colombia as a supplier of marijuana, and a conduit for perhaps one-third of the cocaine moving north from South America.

Like terrorism, this deadly threat may be controllable to some extent by security measures. But it is dealt with most effectively at the source. And unlike the U.S. efforts to stamp out the markets, the efforts of the Mexican government to deal with the source are so shot through with official connivance that even an intensified U.S. drug-enforcement effort could not be expected to have a decisive effect.

There lies the value, such as it may be, of the Reagan administration's retaliatory diplomatic strike. At the very least, the Mexicans can no longer doubt that some responsible officials in the U.S. government and influential members of Congress are up in arms and ready to go public with a case they have been making with increasing vigor but with somewhat more discretion for some time.

There was no blurring of the gut question when U.S. Customs Commissioner William von Raab and Assistant Secretary of State for Interamerican Affairs Elliott Abrams testifed before the Senate Foreign Relations Subcommittee on Western Hemisphere Affairs two weeks ago. Von Raab spoke of "an ingrained corruption in the Mexican law-enforcement establishment." He said it was "massive . . . all the way up and down the ladder." Abrams said he was aware that the Mexicans would be "angered by this kind of discussion in public [but] they have got to get organized to stop this before it gets too late."

That the administration's performance was calculated for its shock effect is confirmed by the appearance of the same two officials at a closed-door session of the Foreign Relations subcommittee the previous day. The chairman is North Carolina's Jesse Helms, who is not exactly a bellwether of public or congressional concerns. But the full committee's chairman, Richard Lugar, was sitting in, and in no way surprised by what von Raab and Abrams later said publicly. "They had already rehearsed the testimony," says Lugar, and he had to "presume" that they were speaking in a "thoughtful, diplomatic way."

The two witnesses were also speaking in full knowledge of the Mexican government's efforts to pressure Helms into canceling the public testimony. As Helms tells it, he received "three or four calls" from the Mexican Embassy warning that a public hearing would "fracture relations -- that this is too sensitive." Sure enough, the Mexican foreign ministry replied to the administration blast with a formal protest: Mexico "categorially rejects the accusations and calumnies" and demands an explanation for this "libelous" interventionism.

Good. What's needed is plain talk from both sides. One way or another Mexico has to be persuaded that corruption and/or indifference on the part of Mexican authorities is increasingly going to be taken by the United States as an unfriendly -- if not hostile -- act.