THE FEELING IS BACK. It's a feeling people in their 30s grew up with, but for a few years it's been gone. I didn't even miss it until it came back.

It was a powerful undercurrent in our lives, running strongest when we were young. It dragged us down with feelings of fear, of hesitation, of not quite knowing how to plan, of questioning what point there was to our lives, to any legacy we might create.

Ours was the first generation to grow up with the realization that ours might be the last generation. Better dead than red we were taught. "Why not," reasoned a high school friend, confronting the inevitable question young women confront with their first true love. "We'll all be dead from the bomb in a few years anyway."

We grew up with fire drills and air raids in schools, and we learned enough about the bomb to know that air raid drills were probably exercises in futility.Our parents finished off bomb shelters instead of basements. A lot of us went to see "On the Beach" and had our minds branded with what the end might be like. Gregory Peck leaves Ava Gardner and sails home in the silence of nuclear devastation. It was the kind of movie you didn't forget. You just tried not to think about it.

Later came Dr. Strangelove. Sure, the bad guys were still the Russians, they still had the bomb, but they'd had it for a few years without dropping it. Maybe they weren't so evil and crazy after all. Stalin was dead and we weren't. Now, the U.S. dealt with Khrushchev, the jolly bear, a man you could at least talk. We had the hot line between Moscow and Washington. World leaders were beginning to find ways of negotiating with each other, perhaps of even sharing values and ideals. The Russians might not be so bad after all. Dr. Strangelove ended with the world blowing up, but except for that touch, it was a pretty funny movie.

It reflected a new view of life. The Russians and the Americans would continue to build ever more swift and powerful weapons of destruction, but there seems to be a tacit understanding that neither side would use them. That would be folly. Finally, after decades of revolution, we believed that a sane and stable leadership had emerged in the Kremlin, reasonable people who wanted to coexist with us. We felt secure. We worried about inflation, not nuclear war. We worried about our sons' education, not their draft status. We still spent billions on defense, but we could now spend more attention and money on social welfare programs to make a secure country a little bit better place to live in. Peking and Moscow were no longer the enemy. They were places to visit.

Only a few voices, a small chorus of hard-liners, warned that it was a false sense of security. Most of us didn't pay attention. We had had enough of war, and we believed the new Russians had too.

Then the new Russians invaded Afghanistan and President Carter went on television, saying: "My opinion of the Russians has changed more drastically in the last week than even the previous 2 1/2 years." There were the inevitable " "I told you sos," but President Carter was hardly alone in being shocked. The new Russians we found out, aren't much better than the old Russians.

The parallels with the eve of World War II are unmistakable, and we are trying to learn from the lessons of history.No one is talking appeasement. Far from it. President Carter has declared that the troubled countries in the oil-rich Persian Gulf are vital to our national interests. Russian actions there are a direct hit in other words. He wants to resume draft registration and to make a major military committment in that part of the world.

"The implications of the Soviet invasion of Afganistan could pose the most serious threats to world peace since the Second World War," President Carter told the nation Wednesday night.

Maybe the Russians will pay attention to his move and pull out of Afghanistan or at least not advance any farther. We know Hilter did not. He wasn't stopped by appeasement and he wasn't stopped by saber-rattling. He was stopped by nations going to war in what was probably the last globar war in which there were winners and losers.

It's frightening what is going on now, and the feeling that this could be it, the feeling of uncertainty and fear for the furture are back full srength. But there's something more: For a while people who grew up with the bomb were able to live without worrying about it. Obviously, that was a delusion, a false sense of security. But for a while, we were able to look at our children and talk of grandchildren and not have an asterisk in the backs of our minds. No matter how this works out, we won't have that feeling again for a long time. Maybe never again. And that's a little sad.