ANDREAS PAPANDREOU, the democratically elected prime minister of Greece, sure knows how to make it hard for his country's American friends to keep smiling. His latest provocation is the hostile and preposterous assertion that the KAL airliner shot down last year, with the loss of all 269 people aboard, was "on a spy mission for the CIA" and "really did violate Soviet airspace for intelligence-gathering purposes." Mr. Papandreou introduced this canard by stating that "the world" is now convinced of it. Not so. The only people convinced of it are those who -- when it comes to charges against the United States -- do not demand proof and are prepared to accept circumstantial evidence of an especially flimsy kind.

In Mr. Papandreou's Socialist Party, which he was addressing at the time, there evidently are a good number of such people. Optimists suggest nervously that, especially with elections coming up next year, the prime minister is under a certain political obligation to cater to party sentiment to his left, and that he would not say any such thing in a considered state paper. It is not the most satisfactory explanation. Principled differences between allies are expected and need not be concealed. But the matter of whether the United States had dispatched the KAL plane to spy is not one on which there is a principled difference. His spokesman's second-day explanation -- that even though the plane was spying, Mr. Papandreou did not approve of the Russians' shooting it down -- was pathetic.

As it happened, just a few days before the prime minister addressed his party, his government made known that Greece was sticking with an earlier conservative government's decision to take part next year in a new NATO project for electronic surveillance of the alliance's southeastern flank. This development followed Mr. Papandreou's renewal last year of an agreement for American use of what he and his party had called, in campaigning in 1981, the "bases of death." Mr. Papandreou's words, in short, are a lot worse than his deeds: that's a key fact of current Greek-American relations. But there is a certain kind of speech so hostile and reckless that it amounts to an action in itself. It is foolish of Mr. Papandreou to expect, if he does, that the American government can merely keep dismissing it as inconsequential blather that should have no impact on policy.