With Washington threatening to "go to the source" in Cuba and with Moscow hinting it may put nuclear weapons there, it is a good time to check the no-invasion, no-offensive-weapons understanding that ended the 1962 Cuban missile crisis. One thing you find by going back is that Kennedy's and Khrushchev's (still unpublished) exchange is apparently being widely misread by the Reagan administration and by others arguing for a hard line.

CIA Director William J. Casey, for instance, when asked whether the arrival of new MiG23s in Cuba violates the 1962 terms, told U.S. News & World Report on March 8: "Oh, sure it does because the '62 agreement said the Soviets would send no offensive weapons, and it also said there would be no export of revolution from Cuba. The agreement has been violated for 20 years."

On Wednesday, The Wall Street Journal recalled editorially that Kennedy had said after the crisis (on Nov. 20), "if all offensive weapons are removed from Cuba and kept out of the hemisphere in the future, under adequate verification and safeguards, and if Cuba is not used for the export of aggressive Communist purposes, there will be peace in the Caribbean."

The impression is being conveyed that the Kremlin is violating its word by shipping in "offensive" weapons and exporting revolution, and may violate it further by emplacing new missiles. President Reagan, while saying Wednesday night--accurately--that putting missiles into Cuba would be a "total violation," added that "there's been other (unspecified) things we think are violations." All this opens the possibility of dropping the bar against an invasion of Cuba.

Well. The Kremlin is shipping in MiG23s and, unquestionably, is exporting revolution.

But there is no evident basis for claiming that these acts violate the 1962 terms.

To comment on the first, I rely on a summary of the public record by Raymond L. Garthoff of the Brookings Institution, a retired diplomat, in the Political Science Quarterly, Fall 1980.

The "offensive military equipment" that Kennedy pronounced unacceptable on Oct. 22, 1962, included the ballistic missiles and "jet bombers, capable of carrying nuclear weapons"--IL28s. But Cuba's MiGs of the day (21s) were never declared "offensive," either in their fighter-interceptor or fighter-bomber version. Nor, as the 1962 understanding was updated by word and practice over the years, did the United States ever so proscribe the MiG23s (of both versions) that started showing up in 1978: they were few and not fitted out for nuclear arms.

As for the suggestion that the export of revolution violates the 1962 terms, the Soviets in 1962 did not forswear revolution. Nor did Kennedy say they had. He did list (Nov. 20) what "Chairman Khrushchev . . . agreed" to do: remove and keep out offensive systems, permit follow-up verification and safeguards. The United States, he went on, agreed not to invade.

Later in the same statement, he added another condition to his no-invasion pledge; it is the one often cited now--"if Cuba is not used for the export of aggressive Communist purposes." But he did not contend Khrushchev had agreed to it. In the next breath he spoke of "subversion from Cuba" as something we would be continuing to try to halt by other means.

If the Soviets keep "offensive" weapons out of Cuba, is the United States still bound by its no-invasion pledge? The pledge was limited by Khrushchev's undertaking to arrange verification and safeguards. The Kremlin never delivered.

In 1970, however, Henry Kissinger, thinking to button down the Soviet no-offensive-weapons pledge, "reaffirmed" (as he put it in his memoirs) keeping hands off Castro. Inexplicably, he dropped the verification and safeguards condition, asking nothing in return. On Sept. 25, 1970, moreover, briefing the press about a threatened Soviet sub base at Cienfuegos, he indicated that the Kennedy no-invasion condition-- that Cuba not be used to export aggressive Communist purposes--had no standing.

Is all this academic? I think not. Soviet- American understandings or agreements are special, to make or to break. The 1962 understanding embodied the vital if not the supreme interests of both sides. Its collapse or even its substantial erosion could have the most dire consequences. Tampering with the terms, or suggesting that the other side is, is playing with fire.