Now that President Reagan has visited the Bitburg military cemetary where members of Hitler's infamous SS are buried -- and by that visit given new grief to Jews who lost 6 million relatives to Nazi atrocities -- he and his advisers must try to understand why the move evoked such a wave of unhappiness from people in all walks of American life.

The Nazi reign of horror unleashed barbarism on a scale never seen before in this century. It was a time of genocide; of scientific experimentation on prisoners, with even children used as guinea pigs in laboratories; and of bodies piled in ovens.

There could be nothing wrong with Reagan visiting a West German cemetery filled with the graves of German citizens or even soldiers killed during World War II, but a trip to one containing graves of SS members was reprehensible.

"To the survivors of the holocaust whose terrible suffering has made you ever vigilant," the president said yesterday, "many of you are worried that reconciliation means forgetting. I promise you, we will never forget."

But, contrary to the hopes of his advisers, those words cannot quell the pain and controversy Reagan's visit has caused. For many Americans are so embittered by what the president did that they cannot appreciate what he said.

Among those who are in doubt about his sincerity as West Germany and the United States celebrate 40 years of peace are black veterans, who played an important role in the war and saw firsthand what the SS had helped to bring about.

Black soldiers helped to liberate Jewish concentration camps in the waning days of the war, a fact many historians fail to mention, and thus were among the first to see the horrors of concentration camps.

Among them was Paul Parks, now owner of an engineering firm in Boston, who entered Dachau in 1945 with 137 other black soldiers. In a telephone interview last week he recalled that shocking day.

"We thought it was a military camp when we broke through the gates," he said. "It was enclosed by barbed wire. When we broke in, we suddenly realized it wasn't a military camp because there were no soldiers.

"There were dead bodies piled along one side of the fence. Piles of people's gold teeth were near the bodies. I've always remembered those piles . . . ."

Recovering from the initial shock, Parks and members of his unit proceeded through the camp.

"We saw the barracks that housed the prisoners, and when they saw us their faces lit up. They were these emaciated people in striped uniforms. I did not know what a concentration camp was, but I saw women and children. Then a rabbi came out. I asked him how they came there. I told him I was confused.

"The rabbi told me the only reason they were there and being treated that way was because they were Jews. I told him I couldn't believe anyone who seemed to be as civilized and as highly intelligent as the Germans could do these awful things to people. It was the most awful thing I'd ever seen. I didn't know how to deal with it."

Parks sat and talked with the rabbi, and eventually, he said, "I understood. I told the rabbi that for 300 years my people were born and worked until they died, and others came to take their places. I told him my people went through a holocaust for 300 years at the hands of people who were also supposed to be highly civilized."

I also discussed Reagan's trip to Bitburg with another man who understands history and the lesson it holds for the future, the Rev. Jesse Jackson. Today Jackson begins a three-day visit to Europe to mark V-E Day; he will address the European Parliament a day before Reagan.

"Hitler was not a communist or a Marxist but an unbridled racist," said the former candidate for the Democratic presidential nomination. "That is the lesson. We are blind-sided by racism and obsessed with communism."

If the president was truly concerned with reconciliation, he would not have tried to revise history by laying a wreath on the tombs of Nazi special police. He could play a stronger role by laying a wreath at the site of the Sharpeville massacre in South Africa -- or in Beirut.