Back during World War II, a common expression was, "Don't you know there's a war on?" If you were a child, which I was, and asked your parents for anything they didn't want to give, that was the response you got -- and there was no sense in arguing.

Now, though, things have gone totally the other way. No one seems to notice that again there's a war on. It's a small war, a covert war, but a war nonetheless. Only Nicaragua seems to notice.

How else can you explain the persistent sense of shock and outrage that comes over the nation whenever something else is revealed about what the United States is doing in Nicaragua. The other day, for instance, a contra leader, Edgar Chamorro, testified that the CIA told him the United States was out to topple, not reform, the Sandinista government. Chamorro's testimony was played in the newspapers as news when it should have been self-evident. Men are not going to risk their lives to "reform" their government. These are soldiers, not members of Common Cause.

Still, this country's capacity for delusion seems limitless. Almost a year ago, the Sandinistas reported that their ports were being mined. America yawned. But when it was revealed that Americans had assisted in the mining, all hell broke loose. Once again, we were shocked. But, once again, who did we think was laying those mines?

There was a similar howl from both Congress and the public when the contents of that now-infamous CIA training manual were revealed. Everyone seemed surprised. Kill? Me? Perish the thought. But what did we think was going on in the jungles of Nicaragua?

And when the Sandinistas said that civilian officials were being assassinated, who did we think was pulling the trigger? Say what you will about the CIA operative who wrote the manual, he at least appreciated what we are doing in Nicaragua -- making war. That usually entails some killing.

There is something about Nicaragua that produces both bewilderment and a measure of incredulity that a con man would kill for. Take the recent episode of the now-you-see-them, now-you-don't MiG21s. According to administration sources, crates that usually contain MIGs were loaded aboard a Soviet freighter that later arrived in the Nicaraguan port of Corinto. At that, the administration went into its Chicken Little mode, crying "The MiGs are coming, the MiGs are coming." Not since the good old days of bomb shelters and "take cover" had there been such an artificial panic, such a false sense of alarm.

But there were no MiGs. Suppose, though, there were. The official line is that the MiGs would "alter the balance of power in the region." The phrase gets repeated like a catechism, as if saying it over and over makes it true. But Nicaragua is a small, impoverished country that the CIA is fighting out of its petty cash drawer. It's reeling from a combination of the contras and its own economic mismanagement, and it's hard to believe that a few MiGs could alter the region's balance of power. Not only have we turned Honduras into a tropical aircraft carrier, but the United States decides the balance of power in the region. We are in the region, we have the power and we can do pretty much as we want.

You can make what you want of Nicaragua. It is clearly not a democracy in the usual sense of the word, but neither is it your basic communist government. It is something in between -- a disappointment to almost everyone.

But whatever it is, it is not now a threat to other countries in the region. Even American experts concede that the bulk of the arms the Sandinistas have been buying are defensive in nature. The Sandinistas are, as only they seem to realize, fighting a war.

There is little doubt that Nicaragua is on the Reagan administration's hit list -- reforms or no reforms. The terms "Marxist" and "communist" are brandished in all their 1950s glory, as if we had not learned a thing since then, and as if once you've said them, nothing you say afterward has to make any sense. Little Nicaragua has been puffed up into a hemispheric menace, and we have, covertly and in a rather modest way, gone for its jugular. There's a war on. But only the Nicaraguans seem to notice.