Less than 18 months after sweeping to power on pledges to rid Greece of American military bases and quit the European Community, Prime Minister Andreas Papandreou is poised to collect billions of dollars to do neither.

The Socialist leader's odd-man-out tactics in NATO and the community and the high price that he has demanded to renege on campaign promises have led to frustration among other western governments and within Greece's still fragmented conservative opposition. Many of his most demanding left-wing supporters are angry at the backtracking on international issues, despite Papandreou's success in pushing through some promised domestic social reforms. But with a modest economic upswing expected, and an opposition lacking in organization and spark, Papandreou now seems assured of winning the next elections.

An assessment of the first year and a half of rule by Greece's first Socialist government begins when it comes to personal style with a sense of deja vu. The fiery campaign orator who promised a "contract with the people" appears to have emerged as a traditional Greek political patriarch, keeping power tightly centralized in his own hands.

A bent toward autocratic rule, typified by fancy beach resort weekends, luxury limousines and a penchant for favoring his family with political plums, has brought disillusionment. "He confuses socialism with classic 19th century politics," said Stathis Panagoulis, who resigned from his job as undersecretary in the Interior Ministry and was expelled from the Socialist party for complaining about such abuses.

At the same time, however, Papandreou has managed to begin anchoring this never easy-to-rule country firmly in the 20th century. In foreign policy, he has succeeded in putting Greece back on the map in Greek eyes after a generation of following the American lead.

In return for a Greek commitment to stay in the 10-nation community, the Eurocrats in Brussels have completed a package worth just under $3 billion during the next four years.

Devised in answer to concessions demanded by Papandreou a year ago as his price for not demanding a referendum to remove Greece from the Common Market, that largesse constitutes an extraordinary effort by the Brussels commission, particularly because Greece's creaking bureaucracy has so far proved unable to absorb more than one-third of proffered Common Market aid.

The offer features a special financial and economic package, deemed "a sort of Marshall Plan" by one community official, plus an indefinite delay in applying Common Market rules on tax reform and other basic legislation.

As for the American bases, considered vital for the North Atlantic Treaty Organization's exposed southeastern flank, a general mood of optimism surrounds the discussions on their future despite cautionary warnings from those close to the negotiations that serious obstacles remain. Recent American willingness to raise the U.S. contribution from $280 million to $500 million in the 1983-1984 fiscal year to help modernize Greece's armed forces--if agreement is reached--has notably improved the chances for success, according to politicians and diplomats here.

Such is Papandreou's reputation for quixotic behavior, however, that no one is taking the prime minister for granted when it comes to either set of negotiations.

In the critical issue of Greek relations with arch-rival Turkey, Papandreou recently signaled willingness to resume a dialogue within two months at the level of foreign ministers. Such is his record on that score, however, that Turkish diplomats have expressed private doubts about any substantial improvement in relations.

In domestic policy, the prime minister's reforms, although well overdue, have required little expense.

"He's been offering little presents," remarked Helen Vlachos, publisher of the right-wing daily Kathimerini, "like a husband promising a fur coat or jewelry and showing up with perfume, and not French at that."

Changes range from instituting civil marriage and divorce, decriminalizing adultery, overhauling the antiquated university system and granting of a general amnesty that allows tens of thousands of Greeks who fought on the losing, communist side in the civil war of the late 1940s to return home.

On paper, at least, the prime minister now stands on the threshold of a modicum of economic and financial prosperity, calculated to allow expensive structural changes delayed by a sluggish and vulnerable economy. If the world begins to emerge as forecast from the current recession, improved foreign-exchange income from tourism, shipping and remittances of foreign-based workers can stimulate the thin and lackluster economy.

Such a recovery would bolster Papandreou's political position. His party already controls 175 of Parliament's 300 seats and has every reason to feel firmly in control until elections, which could be delayed as late as October 1985.

After granting an average 27.5 percent wage increase last year, concentrated among lower-paid workers, the government clamped on a salary freeze in January while letting prices rise as a way of damping down the economy. Inflation has been reduced from 25 percent to 20 percent annually, the balance-of-payments deficit is down from $2.5 billion to $2 billion and unemployment has stayed steady at approximately 8 percent nationwide.

But for all the talk of streamlining the never impressive civil service, the Socialist government has followed its conservative predecessors in turning out earlier political appointees and naming its own people in large numbers.

More worrying to some observers is the government's apparent sense of insecurity, underlined by the government's ordering of a police, Army and party alert on Feb. 27 when it apparently felt threatened by a possible Army uprising involving monarchists.

Neither Papandreou nor his ministers has explained the alert. In a country where the last military government ruled from 1967 to 1974--the seventh such government in a century--such a stance proved unnerving to many Greeks of all political leanings.

Even highly placed Socialist party officials and well-disposed diplomats--as well as the conservative New Democracy opposition--questioned Papandreou's handling of it.

The late winter and early spring traditionally have been periods of political tensions--partly because senior Army officers come up for advancement or forced retirement then, and the prime minister by all accounts acted on serious information. But there now seems little to suggest that any serious plotting was afoot, in contrast with 1967 when the colonels barely bothered to disguise their intentions before they seized power.

Equally upsetting to many Greeks has been Papandreou's authoritarian attitude toward state-run television, which in his opposition days ignored him and his party.

In recent weeks he has demonstrated his own penchant for interfering with television.

Television simply did not mention a disturbing incident in which a drowned ship worker's leftist friends dumped his body inside the Merchant Marine Ministry to protest alleged lack of safety precautions by shipowners.

And when a prominent right-wing newspaper publisher, George Athanassiades, was killed recently, Papandreou personally intervened to fire a television news editor who broadcast a statement by New Democracy leader Evangelos Averoff suggesting the crime was politically motivated.

The prime minister said that the editor should have waited for a balancing government statement, and later in the evening the television news carried the government version and only referred to Averoff's.

Even without such incidents, New Democracy is divided and scarcely in good health. George Rallis, the former prime minister who relinquished party leadership after New Democracy's 1981 election defeat, has just published what promises to be a best seller entitled "Hours of Responsibility." In it, he condemns Averoff's tactics of allying the party with the extreme right, which, Rallis says, alienated centrist voters and contributed to the Socialists' election victory.