PRESIDENT REAGAN'S decision to go on fighting Congress' attempt to limit aid to opponents of the Nicaraguan government came as no surprise. His policies toward what he perceives as a serious communist threat in Central America have been consistenly hard-line.

Yet in the rest of the Third World, he has shown much greater flexibility in dealing with radical regimes. Whether this has brought results is another matter.

Ronald Reagan came to power partly because he capitalized on what was considered to be Jimmy Carter's soft approach to Ayatollah Khomeini. Yet once in office the Reagan administration began has been relatively relaxed, seeing some value in the regime's fanatical anti- communism. Despite evidence of the Iranian complicity in the terrorist attacks on U.S. Marines in Lebanon, the United States did not retaliate.

Iran has gone its own way, seemingly oblivious to U.S. policies.

In Afghanistan, Reagan has continued former President Carter's policy of giving undercover military support to the rebels. However, he ended the grain embargo against the Soviet Union. To that extent, the policy is softer. Diplomatically, the United States has been passive, leaving the running to Pakistan and the Association of Southeast Asian Nations (ASEAN).

In Vietnam, Reagan has picked up where Carter left off. Although committed to the removal of the Vietnamese-backed regime of Heng Samrin in Cambodia, Reagan has been no more successful than Carter in achieving that objective. For want of an alternative policy, he acquiesces in Chinese support for the deposed murderous regime Pol Pot, which maintains legal title to Cambodia's seat in the United Nations.

Apparently he has decided not to channel arms to the other elements of the resistance led by Prince Norodom Sihanouk and former Premier Sol Sann. This would draw the United States in deeper than it wants to go, and perhaps lead to another U.S. confrontation with Vietnam. Again, Reagan prefers to leave the main responsibility for keeping the pressure on the Vietnamese to local regional powers.

In the case of Libya, Reagan did adopt a tough policy -- initially.

Americans were asked to leave the country, former Secretary of State Alexander Haig cited Libya as a prime example of the dangers of interantional terrorism, imports from Libya were banned and U.S. high technology exports to Libya were prohibited.

In August 1981, two Libyan aircraft were shot down in a dog fight over the Gulf of Sidra. Military aid to Libya's threatened neighbors was increased.

What was the effect? For a year or two in 1982 and early 1983, Libya did limit its military involvement in Chad, apparently put a halt to the targeting of Americans for assassination and stopped attempting to assassinate Libyan dissidents abroad.

But the pause was short-lived. In the summer of 1983, Libya was back in Chad and in 1984 attacks on Libyan exiles began again. There were well-founded allegations of Libyan mining of the Red Sea and an intent to bomb the Aswan Dam.

Meanwhile, American policy makers became less confrontational. It seemed to make little difference. Libyan leader Muammar Qaddafi continued with his maverick behavior.

Southern Africa is something of an exception to the Reagan administration's generally relaxed policies in most of the Third World. There, it has pursued an active policy of diplomatic engagement, working for a settlement in Namibia, a non-aggression pact between Mozambique and South Africa and a cease-fire between South Africa and Angola.

Although the administration toughened the U.S. stance on some points (particularly when it insisted that Cuban forces be withdrawn from Angola as a quid pro quo for the U.N. settlement plan being implemented in Namibia), in other ways it has shown remarkably little idealogical concern about the Marxist makeup of the Angolan and Mozambican governments.

In 1983, relations with Mozambique were restored to the ambassadorial level and the United States encouraged Mozambique to join the International Monetary Fund. The United States has not funded Jonas Savimbi in his struggle to topple the Angolan government.

Can any conclusions be drawn from all this? Reagan's Third World policies, although tougher around the edges, have not, over time, been sharply different from Carter's. Only in Nicaragua have they been continuously confrontational.

Even in its handling of the Namibian question, the U.S. demand that the Cubans leave is not an extreme position, given that Reagan accepts that it has to be part of a deal in which the reason for the Cuban presence -- South Africa's threat to Angola -- is removed and free elections under U.N. auspices are held in Namibia.

At the same time, Moscow's behavior has been extraordinarily subdued in Third World trouble spots during Reagan's tenure. In a strange way, a modus vivendi seems to have been arrived at. Nothing is boiling up or getting out of hand, except perhaps in Nicaragua.

Compared with other recent eras, this is no mean achievement. But what the secret is, is difficult to divine. An approach based on loud rhetoric but a soft glove? Perhaps. Or could it be that Moscow has been too bound up with its own succession troubles and too bogged down in Afghanistan to give Washington the usual hard time?