Anyone who thinks that public opinion doesn't matter in power politics should know better after the events of the past week.
The British had the Falklands affair going for them handsomely until they knocked off an Argentine cruiser 35 miles outside their 200-mile blockade around the islands. World opinion accepted Britain's right, indeed many thought it a duty, to uphold its authority inside that 200-mile circle around the islands.
But the moment the British took action beyond that circle - they were in political hot water. The Soviets seized on it as an excuse to propose taking the Falklands dispute to the United Nations. Leonid Brezhnev called the British action ''colonial brigandage.''
The Irish, who naturally and reflexively seize any excuse to be anti-English, found in it an excuse to back away from Common Market sanctions against Argentina. Britain's continental European allies almost unanimously turned from support for Britain to demands for an end to the fighting.
Britain's case was reasonable. The Argentine cruiser General Belgrano was a big ship of more than 12,000 tons. She carried formidable weapons. She was steaming toward the circle and toward smaller British ships operating inside the circle. Any military commander would want to take the action the British did; protect his ships from the oncoming attack.
The British took that action. The cruiser was torpedoed and sunk. The danger to the British fleet inside the circle was avoided. But the British government was at once put on the defensive in the court of world public opinion, among its European allies, and even in the House of Commons in London itself. British Prime Minister Margaret Thatcher paid a heavy price for taking that crucial military decision on military grounds. It is a startling reminder of how political any military action can be.
The past week's events also illustrated the incalculable nature of military action. By sinking the Argentine cruiser on Monday, the British lost the defending position in public opinion but won further military prestige. The next day they lost military prestige when one of their own ships, the destroyer HMS Sheffield, fell victim to a single guided missile fired by a single plane from a distance of nearly 20 miles. If a single shot of negligible military can knock out a $50 million destroyer carrying a crew of 300 men, how many more big ships belong in modern naval fleets?
There is bound to be a ripple effect from the scuttling of the Sheffield by a single missile. The United States Congress is coming up to a critical decision on further big carriers for the US Navy. The Navy high command still wants the big ships, but many an admiral is saying that the big carrier is just as out of date now as the battleship was from the time US Gen. William (Billy) Mitchell proved that bombs could sink one.
The disabling of the Sheffield is a persuasive argument for those who believe the US Navy should be building many small ''sea-lane-control'' types of vessels rather than more of the big carriers that grew out of World War II.
Another ripple effect from those two days of fighting was a sense of uneasiness felt and recorded in many places when casualties from the sinking of the Belgrano were estimated originally in the ''several hundreds'' and when the British Admiralty said 30 men from the Sheffield had probably been lost.
Public opinion was untroubled when the British regained control of South Georgia without human losses to themselves. One Argentine soldier was reported killed.
But it was different when people began to visualize those ships going down in winter storms after violent explosions. This wasn't a game. It was real war and it made a lot of people uncomfortable, perhaps principally because it was real war inside the Western community that has been relatively sheltered from such unpleasantness since World War II.
There have been plenty of wars in places outside the Western community since World War II, and plenty of bloodshed in civil strife in Central America and, indeed, inside Argentina itself. But this was a little different because it proved that war and the usual cost of war could again occur inside the relatively safe and relatively established Western community. The overtones were disturbing - and of course added to the pressure suddenly building this week on the participants to get back to the peace table.
A side effect has been to smother notice of events in other parts of the world. The Polish government reimposed tight curfews and other restrictions on free movement of people after violent rioting May 3 in many a Polish city, but particularly in Warsaw.
And in the Mideast, Israeli soldiers shot and killed an 18-year-old Arab schoolgirl, bringing to 14 the number of Arab children killed by Israelis since Israel began in mid-March to clamp down on Arab political activities in the occupied territories. Israeli Prime Minister Menachem Begin added to Arab unrest by declaring publicly that he will assert Israeli sovereignty over the whole of the occupied territories at the end of the five-year period provided in the Camp David agreement for establishing Arab ''full autonomy.''
If ''full autonomy'' is to mean administration of Arab affairs by persons picked by the Israelis, not by elected Arabs, it is difficult to see anything other than another five years of unrest and military repression on the West Bank and in Gaza.