WHEN I WAS IN SCHOOL, almost every historical event had to have a moral. The Battle of Trenton, for example, was supposed to prove something about the unreliability of using mercenaries. Any fight worth fighting, we were told, was worth fighting yourself. It is time we relearn that lesson.

The troops Washington vanquished that Christmas night were Hessians paid by the British to fight a war that they themselves thought necessary but for which they had little taste.

Now we have our own Hessians. It is this creature called the all-volunteer Army in which the very poor, the ill-educated, the hapless, the hopeless and, by some accounts, the incompetent, are paid to do the defending the rest of us are loathe to do. Someday soon, they also may be asked to do our fighting.

It may be somewhat extreme to call the volunteers Hessians when they are, of course, Americans. But they are hardly volunteers, having mostly joined the Army not for some cause, but because they needed the money or the job or the training or maybe just to get away from home. Whatever they are, though, they are either mostly poor or without options.

There is a gaggle of reasons to oppose the volunteer Army. Some critics don't like the very idea of it, and some simply don't like the way things have turned out. They complain, for instance, that the level of intelligence is low and that the soldiers have neither the smarts nor the education to work the complicated weapons of modern warfare -- and certainly not to provide leadership. Military men say that the deficiencies of the volunteer Army are well known to both our allies and our enemies, and that the difficulties of working with such men and women is one reason the critically important noncommissioned officers have abandoned the service as a career.

Some of this is technical and complex and hard for the public to evaluate. What is apparent, though, is that the argument over the volunteer Army is not at bottom a technical one, but a political one. The Reagan administraton favors it because it is against the draft. The president opposed the draft during his campaign, and that position was recently reiterated by Defense Secretary Caspar Weinberger.

It's hard to argue in favor of the draft and even harder to argue in favor of one that excludes women. But the one thing it has going for it is the fact that if it works fairly and well, it affects all segments of American society. This is important because it was not until the sons of the middle and upper classes started to die in Vietnam that the middle and upper classes started to question that war -- to ask, in other words, why their kids were being drafted and why they were dying.

That is a lesson worth remembering, particularly at this moment. The Reagan adminitration is arming just about any government, of either the right or the left, that is in any way anti-Soviet. It has a secretary of State who travels the world ascribing to the Kremlin all manner of evil, from terrorism to indigenous civil wars, and who has simplified what used to be a complicated world by seeing things as either pro-American or anti-American. This is a belligerent, aggressive foreign policy, and it may be only a matter of time until the thought occurs to match the Soviet threat -- real, perceived or just plain imagined -- by committing American troops to combat.

This is always a possibility, always an option -- maybe justified, maybe not.Either way, it is a serious step and ought to involve the entire nation. But if the units doing the fighting comprise so-called volunteers, if they comprise the poor and minorities, if their members come from families who do not vote and lack political clout, then they are little more than a different kind of weapon: modern-day Hessians.

Any war worth fighting is worth fighting as a nation. And any large army worth maintaining is worth maintaining with conscripts. We can't pick a fight and then pay someone to do the fighting for us. This is what colonial nations such as Britain did, and this is what we tried to do in the beginning in Vietnam -- and no one wants to go through that experience again. That, after all, is the ultimate moral of history: Those who ignore it are doomed to repeat it.