MOUNTAIN climbers know the thrill of cresting a ridge and having before them, for the first time, an unobstructed view of the summit. Hard climbing still awaits; but the goal is finally in sight.That elation - combining a sense of accomplishment and heightened expectancy - is what many people in El Salvador must be experiencing after the breakthrough agreement last week toward ending the nation's 11-year civil war. Also encouraging was the conviction Saturday of a colonel for the 1989 murder of six Jesuit professors at San Salvador's Central American University, though many suspect that the higher-ups who OK'd the murders are going unpunished. The United Nations-mediated accord signed in New York by President Alfredo Cristiani and the leaders of the five main guerrilla forces resolves a number of the thorniest issues dividing the combatants. A cease-fire could be negotiated before the end of the year. That would end the war; it would remain for Salvadorans to achieve peace. The land is deeply divided, in terms both of the distribution of privilege and wealth and of the bitter hatreds aroused by the brutal conflict. It won't be easy to effect far-reaching economic and social reforms or to contain vengeful passions. The military will continue to have an outsized role in the country. Yet the accord brokered by the UN, with the personal involvement in the last days of Secretary General Perez de Cuellar, is most encouraging. It was made possible by significant government concessions to the guerrillas' security concerns, and it reflects a desire among guerrilla leaders to join the nation's political process. The Bush administration also deserves credit for the progress made. It wisely turned from the Reagan administration's obsession with ridding Central America of Marxism by force of arms and made peace the main US goal in Nicaragua and El Salvador. Despite the doubts of skeptics, the administration has steadily backed Cristiani, whom it saw as best able to coax the military into the peace process; yet Bush - with strong prompting from Congress - also has exerted pressure on the Cristiani government to open up Salvadoran politics and curb human rights abuses. The US has provided billions of dollars in military aid to El Salvador in the past decade. Now it must help the country rebuild and assist in the resettlement of tens of thousands of refugees displaced by the war. Today's optimism won't last long without further progress toward democracy and social justice. The Salvadoran government, with the help of the US, must keep up the momentum of peace.