Raul Alfonsin clearly means business. Within hours of his swearing in last Saturday, Argentina's first civilian president in nearly eight years was giving Argentines every indication that he intends to take full charge of their badly demoralized and economically bankrupt nation.
Warning both the much-discredited Argentine military and once-strong leftist extremists that he will brook no nonsense from either, he said in a statement following his inauguration that ''this is a nation where justice and peace must prevail.''
Like a thunderbolt he has embarked on a Franklin Roosevelt-like first 100 days to begin tackling Argentina's political malaise and economic stagnation. In quick succession this week, he:
* Ordered the court-martialing of all members of three military juntas that ruled from March 1976 through June 1982. They are to be charged with violations of the human rights of thousands of Argentines.
* Asked Congress to formally abrogate an amnesty law decreed by the military in September exempting military, police, and other paramilitary people from prosecution for the disappearances, torture, and murder of thousands of Argentines after the 1976 military coup.
* Indicated he would cashier some 50 senior Army, Air Force, and Navy officers as he cuts back on the bloated size of the armed forces and puts the services under civilian control.
* Clamped price controls on a long list of foodstuffs, medicines, and cleaning materials, as well as cigarettes, cement, and school supplies in an effort to slow the nation's soaring inflation, now running at 1,000 percent a year.
These steps in themselves cannot fully control the Argentine military nor bring order to the nation's economy. But they are important - and many feel impressive - first steps.
Argentine commentators, including J. Iglesias Rouco, the respected columnist for the Buenos Aires morning daily La Prensa, warn that Dr. Alfonsin must move quickly while the military is in disarray. And an Argentine politician with a military background says: ''He needs to remember that mad military ambition lurks in the breasts of most officers and these ambitions will fly again unless he quickly clips the wings of the military.''
Those who will be court-martialed include the three officers, led by Lt. Gen. Leopoldo Fortunato Galtieri, who are already under court-martial proceedings for leading Argentina into last year's war with Britain over the Falkland Islands.
The price-control order, first of many expected from the economic team centered around Economy Minister Bernardo Grinspun, ''will work well if followed up by a series of many such decrees,'' said the mass-circulation morning daily Clarin, which warns that inflation is the nation's most important challenge.