MOSCOW -- When Heinrich Groth was growing up in Soviet Kazakhstan in the late 1950s, the children took to the streets each afternoon to play war -- the Red Army against the Nazis. His friends chose up sides anew each day, but without fail, little Heinrich was assigned to be a fascist.

Thirty years later, Groth has given up a career in research biology to live in a tiny, spare double room in Moscow's rundown Hotel Prinimayet, headquarters of a grass-roots movement that has issued an ultimatum that has frightened both Germany and the Soviet Union.

Unless the Soviet autonomous Volga German Republic -- dissolved by dictator Joseph Stalin in 1941 -- is reinstated immediately, said Groth's group, Wiedergeburt (Rebirth), many of the 2.5 million ethnic Germans here will flee poverty and discrimination and return to Germany.

"Our people have no more hope," said Groth, chairman of Rebirth, which says at least three of every four Soviet Germans are ready to move to the land their forebears left, in many cases, more than 200 years ago. "We want to live in a civilized country. We are little more than slaves here."

The future of the Soviet Germans is one of the most difficult disagreements in the reconciliation between two countries that fought each other with arms in World War II and then with ideology for another 45 years until Chancellor Helmut Kohl and President Mikhail Gorbachev agreed on German unification last summer.

At talks in Kiev last week, Gorbachev told Kohl that the Soviets will announce in September whether the Volga Republic will be restored. Horst Waffenschmidt, Bonn's official in charge of the ethnic German issue, said yesterday that Soviet officials "received positively" the German argument that reestablishing the republic would stem emigration.

The two centuries of German life in Russia have been a roller coaster for hundreds of thousands of peasant families who followed Catherine the Great, herself a German, into Russia in search of good farmland. Many settled along the Volga River and the Black Sea, setting up villages in which German language and culture flourished.

For Kohl, the ethnic Germans pose a stubborn political quandary. Germany's constitution guarantees the right of anyone with German blood to return and claim automatic citizenship -- a right fiercely protected by Germany's large and vocal population of "exiles" from territories throughout Eastern Europe that in the past were ruled or inhabited by ethnic Germans.

But a massive influx of immigrants in the past two years from the Soviet Union and Eastern Europe -- along with the task of absorbing millions of eastern Germans -- has prompted many Germans to say they have had enough. They fear not only that the Soviet Germans will move west, but also that a wave of non-German Soviets may take advantage of a liberalized emigration law that takes effect in 1993.

Although more than half of them speak no German, the Soviet-German immigrants keep coming: Last year, 150,000 arrived, three times the 1988 figure. This year's total is expected to be much higher. Waffenschmidt said last week during a visit to Moscow that one-third of the Soviet Germans are ready to move to Germany, one-third want to stay and one-third are watching political developments before making up their minds.

Kohl is determined to halt the flow by pressing the Soviets to make life more bearable for the ethnic Germans. Bonn is willing to help the process with aid for schools, hospitals and social centers, and it is already pumping $110 million a year into that effort.

But Gorbachev, too, faces a hard choice. He has pledged to improve life for ethnic Germans. He recently told activists that before he decides on the request for a new Volga republic, a democratically organized congress of Soviet Germans must present its demands, a process that began at a meeting two weeks ago.

Two years ago, tentative moves toward restoring prewar rights sparked sharp protests from other nationalities living in formerly German territories along the Volga River. Gorbachev backed off, presumably to avoid another open conflict among national minorities.

The German problem presses particularly close to Gorbachev's sensitivities because Russian President Boris Yeltsin has embraced the Rebirth agenda. The parliament of the Russian republic last week announced the reestablishment of an autonomous German region in the Altai area of western Siberia. Yeltsin also favors compensating ethnic Germans for land and businesses taken in Stalin-era purges.

Groth said he would like to force Gorbachev's hand by driving wedges between him and Yeltsin and between the Germans' adopted homeland and their original fatherland. "I know the politicians and people of Germany are deathly afraid of a massive immigration," he said in halting German. "But they can integrate us. They've just swallowed an entire socialist country" -- East Germany.

Despite early support from Bonn, Groth now finds his group labeled "extremist" and "radical" by the German and Soviet governments, both of which have turned from Groth to Pyotr Falk.

Like Groth, Falk is 39, a well-educated Soviet citizen of German descent who feels more German than Soviet. Like Groth, Falk said he grew up being treated like an enemy. Falk, one of two ethnic Germans in the Soviet legislature, faced discrimination despite the fact that he can hardly speak a sentence in German. As a lieutenant colonel in the Soviet army, he said, he was denied a prestigious assignment in East Germany solely because of his background.

But unlike Groth, Falk said he refuses to consider leaving the Soviet Union. He leads the new Union of Germans in the Soviet Union, which has broken from Rebirth and won Bonn's support because it rejects emigration and accepts Gorbachev's promise of a better life.

"Many of us can no longer put up with staying here," Falk said. "But I want to stay because I'm a fighter."

"Two million people can't have one opinion," said Hugo Wormsbecher, a disaffected co-founder of Rebirth and former editor of the state newspaper for ethnic Germans. "We all want to remain German. We just differ over how to achieve it. Groth speaks for the many of us who can't keep quiet anymore, who are exploding inside. But I see it differently: If we lost a million people to emigration, those who remain behind will have lost their community."

Wormsbecher, Falk and others met with Gorbachev for three hours a few weeks ago and said the president is ready to grant Germans self-rule. "We've waited 50 years, but the government is ready to help us," Wormsbecher said.

The early Soviet Communists encouraged the German minority, establishing a Volga German Republic and allowing German-language schools and local governments.

But after the Nazis attacked the Soviet Union in June 1941, Stalin dissolved the Volga Republic and set out to obliterate all concentrations of Germans. More than 800,000 ethnic Germans were banished to sites throughout Siberia. Many perished in labor camps. Many more stayed in remote villages, intermarried and lost their German ties. After the war, Stalin ordered a 20-year ban on Germans moving back to their home villages.

Later, many Germans found friends and relatives and tried to save old traditions. But without German-language schools, ethnic identity faded. In Kazakhstan, where 1 million Germans live, schools require children to learn Russian and Kazakh. Students who want to study a third language may choose English, but in most schools, German is not even an option.

Falk, like many Germans, found that his family's birth records and other documents were lost. He managed to piece his past together, concluding from folklore and traces of old dialects that his ancestors were peasants who came to Russia from northern Germany around 1700.

Groth said the breakaway group of Germans consists mostly of Communist Party loyalists putting their careers ahead of their heritage. He insisted Soviet Germans will pack up and head west whatever the desires of the two governments.

"We are politicized now," Groth said. "The genie is out of the bottle."