Prime Minister Margaret Thatcher and her entire diplomatic and military strategy to regain the Falkland Islands now face their severest test.
Mrs. Thatcher is staking her own political future on a resolute, flag-flying appeal to United States and world opinion that Britain is backing its diplomacy with force to defend universal principle -- as she put it April 14, ''to see whether brute force or the rule of law will triumph.''
Her government believes that time, and most of the world, is on its side. It sees the steady approach of the Royal Navy, the weight of European trade sanctions, and the 10-to-1 vote in the UN Security Council urging Argentine withdrawal all combining to put more and more pressure on Buenos Aires to yield the islands.
The next visit of US Secretary of State Alexander M. Haig Jr. to Buenos Aires , she said, would be ''crucial.'' Argentina, she claimed, had already shifted to a less militant diplomatic line. Her bold words to a packed House of Commons April 14 were designed to push Argentina still further.
To those here who are alarmed at the prospect of war and who urge compromise, Mrs. Thatcher replied, ''It was not Britain who broke the peace.''
To the argument that Britain had no right to oppose force with force, she answered that the right was enshrined in Article 51 of the UN Charter (the right to self-defense).
Her goal is Argentine withdrawal so that the wishes of the islanders can be canvassed.
Yet she and her government now confront a situation where their moral, diplomatic, and military positions may never be as strong again. The pressure is on her to withhold military action for as long as possible.
If President Leopoldo Galtieri refuses to withdraw, or if he is overthrown by an even more jingoistic military ruler, and if an Argentine warship or auxiliary appears in the 200-mile blockade zone around the Falklands, Mrs. Thatcher will be on a knife edge.
''If the zone is challenged,'' she told the Commons, ''we will take that as the clearest evidence that the search for a peaceful solution has been abandoned. We shall then take the necessary action. Let no one doubt that.''
That is accepted by most members of Conservative, Labour, Liberal, and Social Democratic parties here as acceptable tough talk designed to support diplomacy that is centered on the UN Security Council call for withdrawal and talks.
But if the government goes ahead and fires at Argentine ships, or assaults the Falklands with landing troops, British lives will be at risk -- and public opinion in Britain and around the world may change.
A recent opinon poll of just over 1,000 people here showed three-quarters supporting government strategy, but almost two-thirds saying that the deaths of British soldiers or islanders would not be acceptable.
The chairman of the Labour Party, Dame Judith Hart, called April 14 for a ''pause for peace'' -- halting the task force (though not turning it around) to give diplomacy more time to work.
Although her view was refuted by Labour Party leader Michael Foot, he nonetheless called for diplomacy rather than war. A Tory backbencher, Sir Anthony Meyer, said he could not support war even if diplomacy failed.
Britain must also consider the effect of war on the US, on the General Assembly of the UN, and on the third world.
At the moment, London points proudly at the Common Market ban on Argentina's exports to it (one-quarter of Argentina's exports worldwide), and at its strong support from Canada, Australia, and New Zealand. Above all Britain cites the Security Council vote.
Mrs. Thatcher also appealed, between the lines, to American public opinion. She draws attention to increasing Soviet support for Argentina, and British reporters are told that Soviet Navy ships are likely providing Buenos Aires with intelligence information about the British task force. (One report from Washington on British television says the US is now supplying London with intelligence data about Argentine naval movements.)
''Soviet support,'' Mrs. Thatcher said April 14, ''is hardly likely to shake the world's confidence in the justice of our cause.''
Mrs. Thatcher's difficulty is that the Falkland Islands, though inhabited by 1,800 British people, are a long way away. They already depend on Argentina for air and mail links as well as for other services.
Try as she might, it is hard to keep on equating the importance of the islands with Poland at the start or World War II, or to suggest that world peace is at stake in regaining them. Her appeal is to the principle of resisting aggression -- a democracy facing a dictatorship. The strength of that appeal may well be eroded if she actually goes to war.
Meanwhile, British military preparations intensify. London has chartered a container ship to carry extra Harrier jump-jets to the South Atlantic, reflecting both naval worries about fighting over near-Artic waters and Mrs. Thatcher's determination to be seen preparing for a long struggle.
The Defense Department has ironically recalled to service the assault ship Intrepid, which it tried to sell a few months ago as part of its defense cuts. Argentina was one of the countries who showed interest.