THE year 1991 has been a landmark in the search for peace on earth. In at least 10 countries and regions, breakthroughs have been made toward resolving long-standing conflicts.In Cambodia, warring parties are seeking to implement a United Nations peace plan. In Angola, civil war has been replaced by steps toward an election. North and South Korea have signed a non-aggression pact. Conflict in Ethiopia and Eritrea has, for the moment, ceased. (Though it continues in neighboring Somalia.) Those external powers that once encouraged war in Afghanistan are new encouraging a political solution. Peace talks continue in El Salvador. A constitutional conference has been convened to end internal divisions in South Africa. The guns are stilled in Beirut. India and China have met to improve relations. Arabs, Palestinians, and Israelis are talking about peace. In no case has a final resolution of differences between and among parties been achieved. And, as progress has been made in these areas, new threats to peace have arisen in Yugoslavia and in the disintegrating Soviet Union, and deep divisions have yet to be bridged in Northern Ireland, Cyprus, and between India and Pakistan. Such realities, however, do not detract from the remarkable accomplishments of moves toward reconciliation that have occurred this year, symbolized by photos of parties shaking hands that a year ago would not have agreed to be in the same room. Without doubt, the end of the cold war was a major factor in this progress toward peace. The East-West confrontation did not create the conflicts. But as long as the possibility existed of victory with external help, warring parties were not prepared to deal with the causes of conflict. The end of United States-Soviet rivalry reduced the patronage for war, particularly in Angola and Afghanistan. It has led nations formerly assuming Soviet friendship to reach out to other relationships: Syria to the US; I ndia to China. It gave to war-weary peoples a rationale for seeking an end to battles. It gave new strength to the UN and to its secretary-general to mediate conflict. And, finally, the new detente permitted the US to undertake unchallenged peace-making in Ethiopia and the Middle East. In each case, peace is fragile. The Khmer Rouge still haunts efforts in Cambodia. Elections must yet be held in Angola. The nuclear issue remains unresolved between the Koreas. The future of the army remains a stumbling block in El Salvador. The fractious factions in Afghanistan are as yet far from reconciliation. The obstacles to a non-racial society in South Africa remain formidable. And the peace talks on the Middle East have yet to move beyond the preliminary procedural wrangling to difficult issues such as Jewish settlements, the Golan Heights, and, ultimately, Jerusalem. To achieve the initial breakthroughs peacemakers have of necessity postponed some of the toughest issues to the final stages. Neither is the peace in some areas totally benign. Calm has come to Beirut under the pressure of Syrian military occupation. The Tigrean leaders who have replaced the Mengistu regime in Addis Ababa and who currently dominate the Ethiopian government may be stirring new tribal tensions. Peace is strongest where there are no losers. The resentment of those forced into a settlement - the Christians in Lebanon or the Amharas in Ethiopia - can bring new conflict in the future. Further progress on many issues depends on abandoning maximum objectives. Many suspect that the Khmer Rouge in Cambodia sees its cooperation in the UN plan as but a tactical step in its ambition to return to absolute power. In Afghanistan, the Najibullah regime, established by the Soviets, may still hope to prevail over fractious opponents. In Angola, it is doubtful whether Jonas Savimbi is prepared ultimately to share power with the rival Popular Movement for the Liberation of Angola. Parties on both si des of the Arab-Israeli divide will need to compromise on stated objectives if genuine stability is to be established. The hope of those who have worked for peace in these areas is that breakthroughs will create political pressures on the participants to continue the process. The patient endeavors of the mediators have created opportunities. For this the world owes them thanks. Those facing the harder issues ahead will build on their determined efforts.