London — Both military and diplomatic strategies have accelerated over the Falkland Islands dispute. The British government stands firm amid fervent hopes here that war with Argentina can be averted.
On the diplomatic front, United States Secretary of State Alexander Haig's mediation mission to both sides, now under way, is welcomed by London, with Prime Minister Margaret Thatcher insisting that he came to London as a friend and ally rather than an honest broker.
The military strategy behind Britain's diplomacy tightened a notch with the announcement that Britain will enforce a 200-mile radius interdiction zone around the Falklands beginning Monday. British Defense Secretary John Nott warned that any Argentine ship coming within 200 miles of the islands risked being sunk. ''We are earnest and no one should doubt our resolve.''
The move, a virtual blockade, is designed to force the Argentine junta to think again and to yield the administration of the islands back to Britain.
It is also aimed at impressing Argentine public opinion and prodding it into protesting the junta's occupation.
To go along with the military stick, Britain holds out a carrot: ''Take off your men, and we will take part in talks to achieve a lasting solution to the islands' future.'' But Mrs. Thatcher refuses to begin talks before occupation forces leave, and she is reported to have insisted on that to Mr. Haig.
The secretary of state was due to fly to Argentina April 9.
Britain may eventually have to accept a diplomatic solution. One being suggested by several national newspapers and opposition politicians is known as a ''Hong Kong arrangement.'' This would see Argentina take sovereignty and immediately lease back the islands to Britain for a lengthy period.
The islanders themselves have rejected this plan in the past. Many said they would rather leave than live under Argentine rule.
This could change if they are assured that British democracy, way of life, supplies, and markets for Falkland wool will remain.
The announcement of the interdiction zone, made by Defense Secretary Nott to an enthusiastic House of Commons late April 7, means that Britain already has, or is about to have, at least two and perhaps four nuclear submarines on station in the South Atlantic. Covering a radius of 200 miles around the Falklands is too much for a single submarine.
This does not mean an automatic end to Argentine resupply to occupying forces. Between now and April 12, when the zone is to be established, there is time for all kinds of equipment to be shipped and flown from the Argentine mainland to the islands.
Even after the zone is established, Argentine planes would be able to fly in supplies until the British task force came into range and began intercepting Argentine planes with its own aircraft.
The Argentine junta is under notice, however, that Britain has chosen its opening naval strategy: picking off Argentine vessels one by one and thus raising the cost of the occupation of the Falklands to an unacceptable level.
Meanwhile, many hopes here are resting on the shoulders of Mr. Haig. He arrived in London late April 8, saying he had been sent to help find a way out of the dispute.
He went straight into a meeting with new Foreign Secretary Francis Pym and a dinner with Prime Minister Thatcher.
Britain knew Mr. Haig as a forthright NATO commander in chief, and during the Reagan administration he has gained an image of a staunch anticommunist with a short fuse and a streak of emotionalism in a crisis.
Mrs. Thatcher knew he was eager to blunt any chance for the Soviet Union to take advantage of Argentina's need for help.
In talks with Mr. Haig, the prime minister leaned heavily on the need to confront Argentina, in an effort to persuade him to take the British side. She flatly rejected any plan by which Britain would have to accept the presence of occupying forces.
She reminded Mr. Haig that France, West Germany, and Belgium had already promised to support London's call for economic and military sanctions against Buenos Aires by banning arms sales to Argentina.
On the other hand, third-world countries and most of Latin America oppose the efforts of any colonial power to retain its territories.
Britain also knows that the United States cannot burn its diplomatic bridges with an anticommunist country in Latin America second only in size and influence to Brazil - particularly at a time when Washington is trying to prevent a cordon of left-wing influence being drawn across Central America.
As diplomatic and military activity continued, many Britons expressed hope that a peaceful solution could be found.
The mood in the House of Commons has cooled noticeably from April 3, when the rafters rang with bellicose nationalism. It is widely recognized that the naval task force is a diplomatic weapon, its force to be used only as a last resort.
But London works overtime to publicize its readiness to fight if it must. Television cameras and reporters have been permitted to sail with the task force with the idea that daily reports of military drills and training flights will be picked up in Buenos Aires.
Nor is the government displeased at frequent newspaper stories in which retired admirals suggest naval strategies such as attacking the tiny South Georgia Islands first and using them as a base for moving westward to the Falklands. South Georgia is out of range of Argentine mainland aircraft.
The hope here is that the firmer and more resolute British statements and war preparations seem to be in public, the less likely it will be to fire weapons or sink ships.
Although some jingoistic emotion remains, in many British churches and homes prayers are for peace and honor and a settlement acceptable to both sides.Contributor Alexander MacLeod writes from London on the political challenges facing Prime Minister Thatcher:
Mrs. Thatcher, the iron lady of British politics, is going to need nerves of steel - and rock-solid American help - if she is to weather the Falkland Islands crisis and remain prime minister.
That is the assessment of some of her own most devoted supporters. What is worrying Conservative Party stalwarts is not just the risk Britain is taking in trying to face Argentina down with a show of military power. Mrs. Thatcher has made her own political position exceedingly exposed by publicly stating that she is not prepared to contemplate failure.
In the House of Commons, the Labour Party opposition, backed up by Liberals and Social Democrats, has detected the prime minister's isolation.
When the Conservative Party senses that a leader is not performing satisfactorily, it is rare for criticism to be voiced openly. But the question party strategists will continue to ask about Mrs. Thatcher is: Will she be able to lead our party to victory at the next election?
No final judgment is being made at this stage, but a number of significant points are being noted by observers:
* The departure of Lord Carrington at an early stage in the affair has made it more difficult for the prime minister to cope with the crisis. If the present strategy were to fail, she would not have his protection.
* Had Carrington remained as foreign secretary and the task force operation come to grief, he could have resigned. Now pressures for resignation would build up on Mrs. Thatcher instead. The Carrington buffer is gone.
* The appointment of Francis Pym as Lord Carrington's successor is pleasing the Tory party in Parliament, but it may eventually pose a threat to the prime minister. Mr. Pym is seen as a likely Tory party leader if Mrs. Thatcher were dropped.
* The Labour Party opposition has refused to close ranks behind Mrs. Thatcher in a display of national unity. Instead, it has backed the strategy but carefully made the point that it deplores Mrs. Thatcher's handling of the dispute. Aware that she is increasingly embattled, the prime minister is pinning most of her hopes on a negotiated solution, with US help.