London — Britain has won the war. Now it is trying to win the peace.
There is jubilation and relief here at the Argentine surrender 74 days after the war began - crowds singing ''Rule Britannia'' outside No. 10 Downing Street . . . Prime Minister Margaret Thatcher saying that it is now ''GREAT Britain once again'' . . . Headlines June 15 shouting ''We've Won'' . . . A Londoner telling me the morning after, ''It's a terrible price we paid, but I am that much prouder today.''
Yet there is sober realism as well, summed up by a blue-collar worker in Surrey who said: ''I'm glad the fighting is over, but what a tragic loss for so many families. I feel sorry for the Argentine boys too. Why did it have to happen?''
Having lost about 230 men, 5 naval vessels, 1 container ship, 8 Harrier jets, and 11 helicopters, and having spent about $2 billion so far, Britain now heads into a period of postwar complexity in the short, medium, and long terms.
In the immediate short term, the government mood here is tough and unyielding.
Mrs. Thatcher's Cabinet simply does not trust the Argentine junta. Before it breathes easy, the Cabinet is insisting on a total halt to hostilities -- that is, no more attacks to be launched from the Argentine mainland. In case the junta is tempted or decides to hold out for unacceptable conditions, the British are keeping a number of hostages on the islands - commanders and officers. They will be released only when London is satisfied that Argentina has stopped fighting altogether.
For the moment, Britain has a major humanitarian problem to solve - feeding, sheltering, and caring for a far larger number of Argentine prisoners than previous reports had indicated. Mrs. Thatcher told the House of Commons June 15 that 11,000 prisoners had been taken in Port Stanley alone.
Two thousand more were on the West Falkland Island and Britain had already taken 1,800 prisoners.
So a total of 14,800 Argentines, together with an estimated 9,000 British forces, and upward of 600 islanders in Port Stanley all need to be provided for on tiny islands that normally support a mere 500 families.
When captured, Argentine troops in Port Stanley had very little to eat, officials said. Many had no tents or shelter, many were ill from exposure or winter cold.
Food brought through the British blockade by a container ship, the Formosa, on May 1 had apparently long since been eaten.
The immediate British plan is to get as many prisoners as possible onto task force ships. Leaving their arms behind, they will go to neutral countries such as Uruguay and Chile on route back to Argentina.
Many of the Falklands sheep may have to be killed for food, officials indicate, and task force supplies will also be used. Mrs. Thatcher told the House June 15 she felt the United States would be willing to help if necessary with food and transport.
Also in the short term, British administration will be reinstated as soon as all hostilities have ended. Mrs. Thatcher said that British Falkland Islands Gov. Rex Hunt will go back to the islands - but it is understood here that he may not stay long. He is a symbol of colonial rule. The government's aim now is eventual self-government for the Falklands on the pattern of the Isle of Man, with Britain remaining responsible for the Falklands defense.
Mr. Hunt will not carry his ostrich-plumed governor's hat or his splendid uniform or sword. He will act as civilian chief while Britain's commander, Maj. Gen. Jeremy Moore, will be military administrator.
A visit to the islands by Mrs. Thatcher herself is not ruled out here.
The government will push ahead and lengthen the Port Stanley airstrip from 4, 000 to 10,000 feet to take Phantom jets, and other aircraft. It will rebuild homes and farms, construct badly needed roads, and provide quarters for a garrison force of between 3,000 and 5,000 British soldiers.
Here at home, opposition political parties are united in congratulating Mrs. Thatcher for the triumph of her policies and in praising the armed forces.
Mrs. Thatcher and the Conservatives have scored their greatest triumph since they won the general election of 1979.
The prime minister is riding high. Opposition leader Michael Foot paid open tribute to her on the floor of the House late June 14 after her dramatic announcement of an imminent surrender.
Her stature has been enhanced as a bold, decisive, able leader, while the Labour Party is deeply split and the Liberals and Conservatives are languishing because the spotlight of publicity has been turned away from them for so long.
Yet the opposition is pressing for a quick inquiry why Britain was caught by surpise on April 2 when Argentine forces occupied the islands. What happened to British intelligence? Should procedures be changed? Who was to blame?
The main inquiry, to be called by Mrs. Thatcher herself, will be carried out by a small group of eminent men. Under one plan, each of the political parties would appoint a privy councilor and the chairman would be a High Court justice.
A second inquiry is to be held by the All Party Defense Committee in the House of Commons. It will focus on the flow of information from the Defense Ministry to the media.
Until all hostilities had ceased, Britain wants European Community trade sanctions against Argentina to remain in place. Recently, seven EC members joined Britain in extending trade sanctions indefinitely, although Italy and Ireland refused.
Britain is telling Brussels that trade sanctions are needed in this crucial interim period. In theory, the sanctions cut off a fourth of Argentina's exports.
In the medium term, Britain will look for ways to move the Falklands to a form of self-government.
Mrs. Thatcher flatly rules out any Argentine sovereignty or control. She believes Britain has suffered too many losses to allow it. She also dismisses a United Nations trusteeship.
Instead, the British garrison to stay on the island will guard against any future attacks.
The prime minister has not given up hope of a multinational peace-keeping force to include US soldiers, despite a cool US response. Officials say that Britain is prepared to defend the Falklands by itself, but they hope other countries will help.
Officials are sensitive when asked whether the US might not suffer more than Britain in long-term relations with Latin America. In private, they say, a number of Latin American countries will not be sad to see Argentina taught a lesson. Both the US and Britain, they say, may find it easier to continue relations with the hemisphere than many had believed.
Plans are being laid to attact more farmers to the islands, to stop young people leaving, and to diversify the economy.
In the longer term, London stands ready to talk to Argentina about shared commercial development of the Falklands, including oil and gas exploration. But no sovereignty will be yielded. Some backbench members of Parliament urge the creation of a South Atlantic Treaty Organization to parallel NATO and to preserve both the security and the resources of the South Atlantic and Antarctica.
Pressure will now rise in the West for Britain to negotiate a long-term solution for the Falklands in which it will have to yield some degree of sovereignty. Such pressure, officials reply, will be ''stoutly resisted.''
For the moment however, Mrs. Thatcher is adamant that the islanders themselves will be the only people with whom she will talk about the islands's future.
Her attitude is likely to provoke increasing controversy as Western Europe and the US join in trying to soften the British position.