PERHAPS SHE WAS too ambitious, When she left the Commonwealth Conference in Lusaka last month, British Prime Minister Margaret Thatcher took home plans for a comprehensive military and political settlement of the conflict in Zimbabwe-Rhodesia, with the approval of her 38 Commonwealth partners. Last week, her plans seemed to become reality as two delegations arrived for talks in Lancaster House, near Saint James Palace in the heart of London. One group spoke for the Patriotic Front, won alliance of insurgent forces headed by Joshua Nkomo and Robert Mugabe; the other represented the current government, led by Prime Minister Abel Muzorewa and including Minister Without Portfolio Ian Smith.

Unfortunately, the London conference does not seem to be leading up to a peaceful settlement. Now in its second week, talks on the future of Zimbabwe (Muzorewa's government has dropped the "Rhodesia" part) are mired in troubles. Admittedly, the conference got off to a shaky start; just before it got underway, the Salisbury government launched a "pre-emptive" strike against Mugabe's troops in Mozambique, blowing up fuel dumps and radar stations and killing 300. When the Salisbury representatives arrived in London, the Front delegates responded by branding Muzorewa and his associates "criminals" and refusing to shake hands or take tea with them. In return, Muzorewa declared that he would protect his people from "wanton dictators in the making" such as Nkomo and Mugabe. Both delegations insisted they had come to negotiate with the British government, not with each other.

As the conference got underway, there was some progress. A team of skillful mediators from the British Foreign Office, directed by Foreign Secretary Lord Carrington, prodded the two sides to agree on an agenda for discussion. The British hope for settlement along the lines suggested in Lusaka: a cease-fire between the 15,000 troops of the front and Salisbury's 20,000 soldiers and hired mercenaries, constitutional changes to reduce special privileges for Zimbabwe's 230,000 whites and bring black majority rule, and free elections to install a new government under a more democratic constitution. They would retain some safeguards to protect white minority rights, at least for a limited time period.

Gradually, in response to the British proposals both delegations began to indicate their positions on the issues, and as nearly everyone expected, their positions differed drastically. Nkomo and Mugabe wanted to discuss "pre-independence arrangements," or measures to be taken before Zimbabwe can be officially decolonized by Britain amd made independent; in particular, they hoped to concentrate talks on the replacement of current white-controlled police and military forces with their own black troops. But Muzorewa would have none of that. Rather than transfer leadership of the police and military forces to the Front--in effect, ceding control of the state to Nkomo and Mugabe--the Bishop insisted on discussing only moderate "constitutional reforms" without a word about new elections or ending white safeguards.

Both sides took extreme positions from which they claimed there could be no compromise. Muzorewa, just elected as part of an internal settlement between moderates and the white Smith government last April, was apparently unwilling to re-run the election; Nkomo and Mugabe, pressing forward on the battlefiels from bases in Zambia and Mozambique, were seemingly unwilling to exchange a certain cease-fire for uncertain political victory in domestic elections. Muzorewa justified his stand by claiming that the Front leaders were "terrorists" interested in seizing power. For their part, Nkomo and Mugabe called Muzorewa a "white puppet" blocking a switch to legitimate black majority rule.

AND YET, EVENTUALLY, they compromised. Pushed by Lord Carrington, both the Front and Salisbury representatives came to agree on a general agenda last Thursday. Muzorewa made the larger concession, agreeing to consider both constitutional changes and "pre-independence arrangements" at the conference. His decision to discuss the latter seemed to mean a role for the Front. Front forces, for their part, had to compromise too: they agreed to discuss needed modification in a constitition largely handed down from the days of white rule. For a while, it seemed that the London conference might be on the way to achieving an end to 14 years of Rhodesian political strife, seven years of civil, and over 100 years of denial of black rights.

And then the sky began to fall in. Last weekend, Muzorewa hardened his stand by refusing to discuss "issues which are nonstarters, such as our security forces, our military or our police." He complained that "no country anywhere in the world can accept the dismantling of their security services and the forces that maintain the law." These statements, taped in London and broadcast back in Salisbury, seemed to renege on earlier promises to consider a role for Patriotic Front forces in the transition to independence.

To make matters worse, on Monday Smith stirred dissent within the Salisbury delegation by objecting to matters even Muzorewa had agreed to. He called the retention of constitutional safeguards for the 3 per cent white population of Zimbabwe "absolutely vital." It is just those safeguards, of course, which the front-line states of black Africa (Tanzania, Angola, Mozambique, Botswana and Zambia) find objectionable; even Britain, the United States and Muzorewa have conceded that safeguards must be weakened substantially. Many whites from Smith's old Rhodesia Front party also now agree that white safeguards need to be reduced. Smith's comments split the Zimbabwe delegation badly.

As a result of Smith's protest and Muzorewa's about-face, the London conference may have trouble convincing the parties involved to compromise. Some observers say the actions of Salisbury delegation may have foiled the long-term British strategy for the conference. For instance, the The Financial Times has reported that British mediators may be hoping to win acceptance of constitutional changes mainly from the Muzorewa government, in the expectation that Front forces would eventually present unreasonable demands and break up the conference. Then, according to the Times, the Thatcher government in Whitehall could recognize the Salisbury government and refuse to renew economic sanctions against it when they expire in November. If the Front torpedoed the conference, this argument runs, Mrs. Thatcher could explain to her colleagues in the Commonwealth--and the to front line states of Africa--that she had no choice but to recognize Zimbabwe. And if Britain extended recognition, there would be pressure for the United States Congress to follow suit.

DESPITE THE LURE of possible British recognition for Zimbabwe, Bishop Muzorewa may have some good reasons for trying to delay or avoid a constitutional settlement. It seems highly unlikely that the London conference could end successfully without acceptance of British proposals for new elections: and yet elections are about the last thing Muzorewa wants to face right now. The electorate, both white and black, is dissatisfied with his failure to bring a speedy end to the war with patriotic Front forces. Since he took office on June 1, more than 2,000 people have been killed fighting. Hundreds more have died in ground and air attacks launched from Zimbabwe against neighboring countries which harbor Front bases.

Further, new elections would open up a number of controversies left over from last April. Muzorewa's rival Reverend Ndabaningi Sithole has charged that the elections were rigged, and one-time ally James Chikerema has split from the Bishop's United African Council party to form his own splinter faction. Quarrels about whether tribal loyalties unduly influenced some of Muzorewa's appointments are also certain to arise.

The Patriotic Front leaders may also have little to gain from participation in election. If the Front agreed to a ceasefire, it would throw away its strongest card. President Julius Nyere of Tanzania reportedly believes that Nkomo and Mugabe would sweep free elections in an independent Zimbabwe; other observers are not so sure. It is possible that the long-standing military and political rivalry between ZAPU and ZANU, the two wings of the Front, might create confusion if Nkomo and Mugabe were to run together. Tribal rivalries would also play a role, since Nkomo is a Karanga and Mugabe a Zezeru. Besides, accepting a cease-fire without necessarily gaining power through elections would damage morale among Front troops, potentially weakening the insurgent leaders' only sure source of power. Finally, the rising number of civilian casualties brought by the war has led many black villagers to distrust Nkomo and Mugabe.

Whatever the obstacles to a settlement, however, there are also national and international pressures for peace. One way for Muzorewa to regain his domestic popularity, for instance, would be to take credit for a reasonable peace settlement. The African front-line states, whose populations, border areas, and economics have been ravaged by the war, would also welcome peace, but politically they have too much at stake to end the war for a shabby settlement. The present Zimbabwe constitution--in which whites control the courts, military, police and civil service, hold enough guaranteed parliamentary seats to block constitutional changes, and receive an even more disproportionate share of seats in the Cabinet--must be totally overhauled. (A constitutional solution acceptable to the front-line states would undoubtedly bring prizes of recognition and trade, and the lifting of United Nations economic sanctions in place since 1966.)

Margaret Thatcher and her administration also stand to gain considerably from a peace settlement, and to lose if the London conference collapses. For one thing, Thatcher needs a foreign policy triumph to take public minds off the Irish situation, the poor state of the economy, and the harshness of cutbacks and austerity measures imposed by Chancellor of the Exchequer Geoffrey Howe. For another, she would like to associate her administration with a progressive African policy in order to outflank the Labor party, which had been traditionally more interested in the fight against apartheid. Further, the Tory leadership would like to dispose of the whole Zimbabwe issue before its annual party conference in Blackpool next month.

ULTIMATELY, THE FUTURES of black Africa, the Tory government, and the seven million inhabitants of Zimbabwe will by affected by the success or failure of the London conference. Negotiators at the conference, though strained will continue. A possible compromise settlement might include the following elements: some role for the Front forces in the transitional Zimbabwe administration, free elections supervised by Britain or intenational bodies, a new constitution leading to true majority rule and an end to most white constitutional privileges.

By reasserting its historical role as a colonial power in order to assume responsibility for the crisis in Zimbabwe, Britain has attempted to reintroduce the kind of constitutional arrangements which brought independence to many other colonies, notably Sri Lanka and Kenya. While none of these arrangements provided for the kind of white-controlled government found in Rhodesia, some did include minimal safeguards for the white population. In both Tanzania and Zambia, about 1/7 of assembly seats were reserved for whites for a limited time. The task for Britain, and for the delegates to the London conference, is to come up with an equally fair and manageable arrangement for Zimbabwe. It is also needed to insure that Zimbabwe will wind up with a government, not a battleground. A continuing war in Zimbabwe can only worsen turmoil in southern Africa, bring in the South Africans and the superpowers, and aggravate other festering sores in Namibia and elsewhere in Africa.