THE REAGAN administration's hesitation to come to open support of Britain in the Falklands crisis was principled, since it provided a basis to attempt mediation. When Argentina stood firm on its absurd demand to have its aggression ratified, however, the United States had no choice but to come to the equally principled decision it announced yesterday: to back the Brits.

The move could not have been easy for President Reagan, given his earlier efforts to come closer to an authoritarian Argentine regime for purposes of fighting communism in the hemisphere. Nor will the expected enhancement of American relations with Europe altogether compensate for the complications likely to come in Latin America. Mr. Reagan, however, has served the basic principle of world order. He has shown, as Secretary of State Haig put it, that the United States does not condone the use of unlawful force to resolve disputes between nations--even when force is used by a friendly state.

The steps the administration now contemplates to help Britain entail not direct military participation but supplies, plus economic and political pressure on Argentina. At the same time, the president and the secretary of state carefully left open a possible resumption of American diplomacy. The British presumably will not be blind to the possible impact of these latest events on negotiating prospects; certainly the requirement for British flexibility has not evaporated.

Sensible elements remaining in Buenos Aires should see that American mediation continues to offer Argentina its best exit from the crisis. In particular they should note that the administration still holds itself ready to take into account "the interests of both sides" and that the Americans remain well short of endorsing the earlier British insistence that the wishes of the inhabitants, most of whom are thoroughly set in their British ways, must be "paramount."

The prospects of heavy military action are now very strong. The British may apply maximum pressure while Argentina is reeling under Washington's political blow, and they will also want to beat the weather. The Argentine generals, looking for a snap nationalistic distraction, have put their nation into the greatest crisis in its history. As pained and confused as Argentines must feel, however, surely they have among them responsible people who perceive the national interest in moderation and peace.