President Reagan, whose mastery of symbolism is a key to his successful political career, has stubbed his toe on a potent symbol -- the German military cemetery at Bitburg, West Germany, where nearly 2,000 Germans killed in World War II, including nearly 50 members of Adolf Hitler's elite Waffen SS, are buried.

Reagan's controversial decision to visit the cemetery and lay a wreath to commemorate the 40th anniversary of the end of World War II in Europe has outraged many Americans, including members of Jewish organizations and veterans' groups. Many people also are furious that the president earlier decided not to visit a Nazi concentration camp site.

Yesterday, Reagan tried again to justify the visit, partly on grounds that most of the dead soldiers were teen-age conscripts who were victims of Nazism "as surely as the victims in the concentration camps."

The Nazi SS (or Schutzstaffel -- elite guard in German) began in the 1920s as Hitler's small band of personal body guards. By the peak of World War II, its ranks had grown to nearly 1 million. The SS was instrumental in Hitler's attempt to exterminate the Jews and other groups he considered to be inferior.

"They were the most Nazified of all Germans," said a spokesman at the Simon Wiesenthal Center in Los Angeles, "the most dedicated to Nazi principles, the most loyal to Hitler."

Until late in the war, the SS was an all-volunteer unit and criteria for membership were stiff. As standard bearers of the Aryan race, members had to swear they had no Jewish ancestry as far back as the 1750s. Any claim that an SS member was forced to join the unit against his will is "unlikely," says Bob Wolfe of the National Archives.

During the war years, the SS compiled a long record of atrocities. They carried out most of the medical experiments on Jews and other minorities. Some, organized in barracks squads, rounded up Jews in Germany and Eastern Europe and executed them on the spot, sometimes by the hundreds. The most feared of the SS, the Totenkopfverbaende, administered the concentration camps where most of the 6 million Jews died.

The SS, whom Hitler entrusted to carry out the most important of his missions, were deemed more important to the Nazi movement than any other troops. "They helped fight the ideological war," Wolfe said.

World War II specialists estimate that about half of all SS men died during the war. Others -- such as Adolf Eichmann, one of the most infamous members -- were captured after the war, tried, and imprisoned or executed. Some, however, formed organized groups and meet for reunions in Germany and other countries.

The town of Bitburg was a major tank staging area for Hitler's last great offensive of World War II, the German surge through the Ardennes into Belgium, which came to be known as the Battle of the Bulge. The Germans buried at Bitburg died in that battle, and the SS soldiers there were mostly from the 1st SS Panzer Corps, a crack tank unit that spearheaded the offensive.

The battle began on Dec. 16, 1944, when a German army of 250,000, supported by tanks and field guns that had been secretly assembled in the dense woods on the German-Belgian border, burst out of a dense fog that hindered Allied air support and surprised the thinly stretched Americans holding the sector. Some of the U.S. units were recuperating from heavy losses, some were new recruits being phased into combat in a supposedly quiet zone.

Hitler's desperate hope was to demoralize the Allies, possibly capture the major supply port of Antwerp and negotiate a separate peace on the Western Front, which would free him to concentrate on stopping the Soviet Red Army on the east. The offensive enjoyed considerable initial success because of surprise and the Germans' concentration of forces. But by Christmas Eve 1944, the German commanders knew they had failed. By early January the Germans were retreating homeward.

It was the largest pitched battle fought by a U.S. army. More than 600,000 Americans were involved, suffering 81,000 casualties, 19,000 dead. The Germans suffered more than 100,000 killed, wounded or captured, lost an estimated 800 tanks, and were in effect finished as an effective army.

According to German press accounts, most of the SS men buried at Bitburg were 18- and 19-year-old conscripts. But some were officers and noncommissioned officers in their 30s and could have been among the most fanatical of Hitler's supporters.