Sometime within 1987, Greece's Socialist government will have to make a choice that will determine the quality of its future relationship with the United States and to some extent with its European Community partners also.

The touchstone question is one of conflicting political and economic interests. It concerns the future of the U.S. military bases in Greece after the current Defense and Economic Cooperation Agreement expires at the end of 1988.In both its 1981 and 1985 election campaigns, the Socialist Party of Andreas Papandreou, the prime minister, pledged that the bases would close, and Mr. Papandreou still insists that this position is unchanged.

But in his more recent pronouncements, he has been careful to leave the door ajar. If the United States wants negotiations on a new DECA, he says, it has only to ask for them.

Officials in Athens expect a U.S. request fairly early in 1987.

It has been brought home to the government that for budgetary reasons the United States will need to know one way or another before the end of 1987. If the bases are to close, funds for their transfer - probably to Turkey - will have to be provided in the budget that goes to Congress in January 1987.

Washington would probably be prepared to haggle well into 1988 over terms and payments, provided that there had been an agreement in principle that the bases would remain undisturbed for as log as the negotiations continues and for a reasonable time afterwards if there was no final accord.

Though U.S. officials are unwilling to describe the bases as the determining factor in Greek-U.S. relations, they concede that the relations would become sharply different if the bases were ordered out. In those circumstances, the government would then:

* Face the immediate problem of replacing the considerable U.S. military aid linked with the presence of the bases, at a time when the economy is already under strain.

* Be unable to count on continued congressional insistence on a strict numerical balance in military supplies to Greece and Turkey, which the Reagan administration in any case opposes, in view of the enhanced importance of Turkey to the Western alliance.

* Have to accept the loss of the substantial benefits contained in a Defense Industrial Cooperation Agreement signed recently with the United States. Though the agreement has an initial five-year validity, it contains a specific provision for premature termination if the bases leave.

The obvious effects on Greece's position in NATO also would have a spin- off in the EC, if only because of British and West German attitudes. Though the government is angered by suggestions that it is playing a "Third World role" in both organizations, there is growing impatience in the EC over the contrast between its continuous demands for special treatment and its self- exclusion from EC political decisions on such issues as the confrontation of ''state-sponsored terrorism."

There also is a question over prospective U.S. investment in Greece, discussed in some detail last May during a visit to Athens by H.P. Goldfield, the assistant secretary of commerce for trade development. Despite encouraging noises, nothing has yet materialized.

There is no direct connection between investment and the bases. But Greek businessmen draw one with the overall economic and business climate, which has not so far been sufficiently encouraging and could only worsen if the step-by- step approach to improved Greek-U.S. relations effected over the past year were thrown into reverse.

A pointer to coming development should be provided in the next few months, when it will be made clear whether Mr. Papandreou will receive the official invitation to Washington for which the Greeks are pressing. The government says the visit is "a given fact," but there has so far been no actual invitation even without a date.

Both the conservative New Democracy main opposition party, which describes the bases as essential to Greece's own defense interests, and the Communists who want them removed, maintain publicly that Mr. Papandreou already has decided they will have to stay.

They say the government's concern now is to find a face-saving formula that will minimize the risk of defections from the ruling Socialist Party.

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