THE CEASE-FIRE agreed upon yesterday by the opposing commanders at Port Stanley gives hope that, mercifully, the war of the Falklands may be at an end. For all that its denouement was eclipsed by another battle, for both combatants it was an extremely costly struggle. It caught up the prestige of the leadership and the spirit of the people in ways that the two countries will be discovering for some time.

The fighting arose from a pitiful mutual misperception. The British, who had dragged out negotiations aimed at shedding this colonial relic for years, did not think the Argentines were serious about getting the islands back. The Argentines, who had felt a consuming nationalistic passion for decades, did not think the British were serious about keeping them. The Argentines compounded their original error by counting on the United States to overlook their aggression of April 2 and to evade choice between its European and Latin alliances. The British compounded theirs by removing their earlier concessions from the table so that Argentina had no alternative to fighting on but to surrender.

In the end, the British military's professionalism and resources, brought to bear against an army that previously had fought only its own citizenry, told. A blockade sapped Argentina's strength, and Buenos Aires could not convert expressions of foreign favor into real sinew. The two forces had occasion to make some sobering tests of modern weaponry. Neither was short on heroes. But it was never a battle the junta could win. The pope's visit to Buenos Aires, as the final assault began, let Argentina paint its surrender in the colors of "peace."

Will reconciliation eventually come, or will there be further battle? No student of Argentina's dark history can doubt its capacity to generate the myths essential for continuing the struggle. (Not the least of these will be the United States' "stab in the back.") Britain's postwar policy, however, will surely make a difference. To justify her country's sacrifices, Prime Minister Thatcher made a series of statements which, if they are now confirmed, virtually ensure that Britain will have to continue indefinitely to divert a huge part of its NATO resources to defend the islands.

Britain fought for a principle. Its challenge now is to conduct a policy that does not require it to fight again for the same principle. At the least, the British owe it to themselves to let passions cool and to savor the fruits of peace while they contemplate the future of the islands they have so arduously regained.