For US and insurgents, there are gains and perils in a partnership. For the US, backing insurgents is a cost-effective way of rolling back Soviet influence in the third world. For some rebels, US arms have provided that extra edge over the enemy. But others, notably the contras, have found that infighting in Washington makes US support undependable.

The final tally on the ``Reagan doctrine'' - supporting a variety of third-world insurgencies against leftist regimes - is not in. But it is not too early to draw some tentative lessons. The experience in four areas - Afghanistan, Angola, Cambodia, and Nicaragua - indicates that as long as Washington's goals are explicit, practical, and enjoy a political consensus at home, the insurgency tactic can be effective. If these elements are not present, there can be a debilitating domestic political toll.

But an insurgency is by definition a dirty war, in which civilians suffer disproportionately. If American assistance to the insurgents had not been given, this toll would undoubtedly be lower - perhaps significantly so. But the inevitable price would have been to concede outright victory to the Soviets in at least the short run.

And ironically, the cases in which de-escalation is most visible are those in which the Soviet Union has the most influence. Moscow is the power that has the political will to produce flexibility, not Hanoi or Havana, although theirs may be the troops directly backing leftist governments. Any realistic prognosis must take this factor into account.

The benefits to the US:

The support of anti-communist insurgencies has been seen by current United States' policymakers as a middle ground between passive acceptance of a series of communist takeovers and the direct engagement of US troops.

Insurgency support is cheap. A pure cost-benefit analysis shows that a relatively minor input in terms of resources, money, and training can deal a blow to governments within the Soviet sphere of influence. According to several ex-intelligence officers, start-up insurgency support often requires only between $10 million to $15 million per year. The highest sums reportedly given the Afghan resistance in recent years are only in the range of $350 million a year. To deploy US troops for the same end would be politically impossible, militarily foolhardy, and monetarily prohibitive.

What insurgencies need to succeed:

Successful insurgencies require identification with the majority nationalist sentiment in the disputed country. If not, they will be perceived by the populace as a foreign creation, and not enjoy the mass support required to succeed.

The Afghan guerrillas clearly enjoy such identification, even if their countrymen's loyalties are divided among various resistance organizations. But the contra rebels fighting the Sandinista government have never developed a similar political base or identification with Nicaraguan nationalist sentiment, particularly in urban areas.

Active support from regional states that share most of the insurrection's goals is also key. The Afghans have Pakistan and Iran; the National Union for the Total Independence of Angola (UNITA) has South Africa and Zaire; and the Cambodian resistance has Thailand and China. The Nicaraguans, while they do have US-fostered sanctuary privileges in Honduras, have not received a similar level of commitment from regional partners. The provision of safe havens, logistical support, training, and intelligence from such states is essential. The goals of most regional states also tend to be closer to the insurgents' aims than to the US's East-West agenda. And they are, of course, more immune to the unpredictability of Western political processes.

Advanced weapons help

A single battlefield technological advance can profoundly affect the course of an insurgency. Government and Soviet-bloc forces were able to operate in the air with relative impunity in the first years of the Afghan and Angolan counterinsurgency campaigns - until introduction of the US-made, hand-held Stinger missile.

According to former National Security Council official, Christopher Lehman: ``In Afghanistan, it turned the military tide. Once the [Soviet] Hind helicopter was neutralized, the strategic advantage on the ground turned to the mujahideen.''

According to a knowledgeable source who witnessed the Stinger's impact in Angola: ``It was a tremendous morale booster for the UNITA troops. Before its introduction, the Hind helicopter inspired fear and helplessness. This changed instantly, and the feeling of security and even smugness among the troops was overwhelming.

``UNITA troops intercepted radio conversations by Angolan and Cuban pilots expressing fear of the Stinger missile,'' this source continues. ``Pilots often dropped their bombs miles away from the Stinger. Captured Cuban pilots confirmed their `hearty respect' for the Stinger in interviews with Western journalists.''

Perils of dependence on Washington

US involvement in third-world turmoil has proven fickle. It has difficulty sustaining effective support to an insurgency, when there is no clear and largely open domestic consensus favoring such activity. A consensus is present on Afghanistan, but definitely not in the case of Nicaragua. The historical record is a warning to any insurgency not to depend exclusively on Washington for discreet support over the long haul. And as Honduras may be discovering, it is also a flashing red light for any regional state considering association with Washington in such activities.

There is inevitably a distinction between US goals and those of each of the insurgencies being supported. Although sympathies exist for the insurgent goals of a total takeover, Washington's interests are basically satisfied by seeing the Soviets out of Afghanistan, the Cubans and Soviets out of Angola, and Vietnamese out of Cambodia.

Increasing frictions can be anticipated with the insurgencies and their regional supporters, as Washington seeks to compromise short of the achievements the revolutionaries seek. Domestically, decisions not to support insurgent goals of total liberation will inspire strong conservative condemnation of a ``sellout.''

Maintaining secrecy about insurgent support has also proven impossible. In a robust democracy such as the US, there are too many conflicting political agendas which benefit by going public on such contested issues.

Post-insurgency problems:

Once the rebels are seen as having achieved Washington's goals, the issue of continued support or demobilization comes up - especially when the ultimate insurgent goal of seizing the reins of power has not been obtained. Both UNITA and the mujahideen will probably see their fighters absorbed in the post-war government structure. But should the Khmer Rouge be left out of any Cambodian settlement, their absorption, demobilization, or military defeat will prove difficult.

And, being in large part a creation of Washington, finding a way to peacefully reintegrate former Nicaraguan rebels into an appropriate society could become a significant burden on the US government.

The future

The unique circumstances which caused the Reagan doctrine may not recur soon. With notable exceptions such as El Salvador, there are relatively few leftist insurgencies under way in 1988.

Most of those that do exist - most noticeably in Peru and the Philippines - are not closely linked to the Soviet Union. Moscow seems to have recognized the futility and cost of third-world commitments to governments that are vulnerable to continual and costly combat fueled by the West.

According to former KGB officer Stanislav Levchenko, ``Many of the cadres in national liberation movements which were trained in the Soviet Union during the 1960s have been killed or discredited. The human infrastructure for carrying out armed leftist revolution in the third world is no longer readily available.''

In the words of one senior American ex-official: ``The Reagan doctrine has fulfilled its historic mission. Once the Brezhnev doctrine proclaiming the irreversibility of Soviet advances had proven fallible at least once, and is being disproven elsewhere, the attractiveness to Moscow of such expansionist policies plummeted dramatically.''

We may, in short, have witnessed in the Reagan doctrine a phenomenon of the 1980s which will not be required after current hot-spots cool. In the point-counterpoint of international strategy, both superpowers have won and lost at the game of insurgency. And both have come to understand the vulnerability of shaky third-world client-states to destabilization efforts of the other.

For the West, this knowledge has been clear since the start of the Cold War. For Moscow, it may have been the events of the past half-decade, including the Reagan doctrine, which brought the awakening.

Last of three articles. The previous two were published Oct. 14 and 17.