Press comment is building on whether the United States has taken the appropriate stance for a great, peace-loving , democratic power in the matter of the Falkland Islands. To remove any doubt, the US needs to reinforce the attitude conveyed by reports of President Reagan's laudable last-minute effort to persuade Argentine President Galtieri not to invade the islands. In a long telephone conversation Mr. Reagan was said to have indicated the gravity of US opposition to the use of force by telling General Galtieri that such an invasion would wreck the friendship between Argentina and the United States.
What has to be dispelled now is an impression of retreat from this firm stand once Argentina's aggression had taken place. Whatever the rights and wrongs of the territorial dispute between Argentina and Britain, the Argentine resort to arms cannot be condoned. Indeed, the US State Department did deplore it, and the US supported the United Nations Security Council's resolution demanding withdrawal of Argentine forces.
But there was no evident follow-through on Mr. Reagan's warning that Argentina would damage its relations with the US by the use of force. There was no US call for sanctions against the aggressor, which incidentally had not honored the grain-embargo sanctions which the US did impose against another aggressor, the Soviet Union. Mr. Reagan told an Oval Office press conference that the US was friends with both sides. A US representative in the UN debate said the use of force was regrettable but characterized the situation as a quarrel between friends of the US.
One indication of Washington's apparent insensitivity to Argentina's lawlessness was that it left the UN debate to a deputy rather than to UN Ambassador Kirkpatrick herself. It even failed to cancel the ambassador's appearance as guest of honor at a gala dinner given at the Argentine Embassy on the previous evening - last Friday, the day when the invasion took place. Among other US officials present was Walter Stoessel, second only to Secretary Haig at the Department of State.
It was as if the Reagan administration were still caught up by its recent efforts to woo Argentina's repressive regime - for example, to get Congress to lift the ban on military aid. It seemed unable to shift gears when that regime once more exposed its unworthiness. Thus the administration lent weight to the argument that Argentina had been tempted by Washington smiles to think it could more easily get away with aggression now than in previous years.
President Reagan should have given the lie to any such notion by his phone call to General Galtieri. Since the general went ahead anyway, Mr. Reagan has every reason to let him know that Argentina can expect no more friendly support from Washington without radically changing its present course.
Not that the US should be making the Falklands its own problem by offering promises or threats to one side or the other. Secretary Haig's present mission of providing ''good offices'' will be most effective if it helps to establish a climate in which both sides can themselves find acceptable steps toward agreement. For this purpose he can be evenhanded and objective as between all the claims and counterclaims without ever feeding the image of a US that is evenhanded toward aggression.