SIR JOHN HACKETT, in a lecture on "The Profession of Arms," described the system in Sparta. A male child was evaluated in infancy by a council of village elders. If the babe was deemed unfit, he was tossed in the bush to perish. If he passed muster, he was left with his mother for seven years. The state then claimed him and for 13 years prepared him for military service. From the age of 20 until the age of 60 (or until his death or disablement), he was a soldier.

Through subsequent centuries, societies devised various systems to ensure an adequate supply of warriors. Some cultures decreed that warring and hunting would be the only male occupation; admission to that company was the adolescent's rite of manhood. In other cultures, soldiers were bought by the king's shilling or by the promises of booty or status or immortality in another life. Impressment and conscription were employed.

Many of these methods have been used in the United States, and today we find ourselves with a large military establishment that, in one major respect, is unlike any we have had in the past. It is a professional establishment, an establishment of careerists. We call it a "volunteer army," but that euphemism obscures the reality. The 2 million Americans now under arms have chosen an occupation. Many opt out after a few years. But hundreds of thousands are in for their working lives.

It is one of those professions, in Sir John's words, "in which what is demanded of those who pursue them cannot be entirely regulated by contracts between men. The compulsions exerted in these occupations arise mainly from the task itself."

Priests, doctors, lawyers, teachers and farmers are of that company; perhaps journalists, too. But the professional soldier is set apart from them by the "unlimited liability" he assumes. In plain English, one of the occupational requirements is that he be prepared to die or be maimed in the performance of his job.

Every American should be conscious of that "unlimited liability" as the professionals of the Marine Corps return to Lebanon this weekend. Some of them may be killed; some may come home without arms or legs. But that goes with the job. That is what we pay them to do and to risk. It is the profession they have chosen.

If we could be clear-eyed about all this, we could put aside some of the fears and hysterics that arise at the prospect of military engagement. Too often that is not the case.

Just a year or so ago, there were outcries and tremors at the discovery that some American military advisers were carrying rifles in the bush of El Salvador. This summer, when the Marines were first introduced into Lebanon, they were stripped of their normal arsenal, were forbidden to insert ammunition clips in their weapons and were ordered to flee for home if they were fired upon in anger.

There are intellectual and sentimental rationalizations for such silliness. The shorthand phrase is, "No more Vietnams." But phrase-making ought not be confused with logical thought. The death or wounding of military advisers in El Salvador or of Marines in Lebanon is no necessary or even proximate cause for war, any more than the death of policemen in a riot is cause for civil war. That is simply the price we as society and they as soldiers or policemen pay for the maintenance of order and the pursuit of civilian objectives.

Casualties in the professional ranks are not the problem. The problem lies in the political and public passions casualties arouse. There are always demagogues who will wave the bloody shirt and cry out, "Remember the Maine!"

One need not doubt that if and when casualties occur among the force in Beirut there will be hawkish rhetoric as well as whimpering. There will be tearful interviews with relatives and friends and television footage of prone bodies. But it need be nothing more if we have the maturity to recognize and accept the function of the professional military force we have created in the United States.

War-making is the last and most objectionable of its purposes. Its real task is peace-keeping, and if in the pursuit of that purpose casualties are incurred, so be it.

The professional soldier accepts the "unlimited liability" of his calling. Society should accept it with the same grace and stoicism. If we cannot, we should disband the force and return to a civilian militia.