SUPPOSE Israel were located right next to Germany, not across the Mediterranean. Suppose the German economy collapsed, and millions of Germans decided to emigrate to Israel. And suppose that this scenario had taken place back in the 1950s, with the memories of Nazism still fresh. That will give you an idea of what may be about to happen between Russia and Poland.Soviet law remains far from guaranteeing freedom of emigration, but it is painfully lurching in that direction. The more Moscow approaches the ideal of free emigration, the more paradoxes confront Western leaders. NATO countries are already rebuilding the Iron Curtain in the form of strict restrictions against East European immigrants. The chief victims of these restrictions will be the Russians. Odessa Jews will be welcome in Israel and Volga Germans in Berlin, but ethnic Russians will have nowhere to go except the one country that has neither natural nor military barriers to stop them. Unfortunately, that country happens to be the most anti-Russian in the world: Poland. Poland has one of the most homogeneous populations in Europe. In 1939 Poland was only two-thirds Polish, with large minorities of Jews, Germans, and Ukrainians. Over the next decade, nearly all these minorities were uprooted or murdered by Stalin and Hitler. Today's Poland is about 95 percent Polish and Roman Catholic. The Poles' struggle against Soviet imperialism relied heavily on ethnic and religious solidarity, including bitter memories that long predate the Bolshevik era. Every Pole knows how 18th-century Russia forcibly partitioned his country, and how 19th-century Russia oppressed Poland more harshly than any other province. The 20th century, of course, brought greater horrors. Russia's Bolsheviks invaded Poland even before winning their own civil war. Russians have bitter memories of their own. They recall how Polish bishops suppressed Orthodox Christianity at the 16th-century Union of Brest and how Polish troops seized Moscow during the 17th-century Time of Troubles. The first head of Lenin's secret police was a Polish-born revolutionary whom the KGB still honors as its founder. Very often, people just emerging from oppression find it hard to remember that no nation has a monopoly on suffering; neither Poles nor Russians are likely to prove exceptions. Russian immigrants will be about as welcome in Poland as Russian soldiers have been for the last 50 years. But now it is the Russians who will be defenseless. Contrary to some predictions, President Lech Walesa has so far proved to be a responsible leader committed to keeping Poland on course toward economic freedom. But he may soon find himself outflanked by demagogues appealing to Poles' rawest hatreds. Even if they avoid the ugliest forms of ethnic politics, the Poles will be hard-pressed to provide the jobs that the Russian newcomers will need. One Polish official says that Russian immigrants "could finish off our economy in three or four months." IF so, the damage will be felt far beyond Warsaw. With its bold steps toward the free market, Poland is a crucial test case. If the Poles can deliver high living standards when Moscow's half-steps have failed, others will imitate them. If not, reform will look less attractive to other former Warsaw Pact countries. Even more ominously, anti-Russian riots in Poland could give Kremlin hard-liners an excuse for keeping Soviet troops there - and perhaps reinforcing them. Why not just seal Poland's eastern border by force? Americans should know how hard that is to do - look at the Rio Grande. And even if Poland were willing to shoot Russian immigrants on sight, the effect would merely be to transfer the problem to the westernmost Soviet republics - Lithuania, Belorussia, and the Ukraine - inflaming their own anti-Russian passions. The more harshly the non-Russian republics treat ethnic Russians, the less willing the Kremlin will be to let any of those republics secede. Should we increase United States financial aid to help Poland cope? That response simply misses the point: Russians are culturally and politically unassimilable in today's Poland. But ironically, these Russians would be economic and cultural assets, not burdens, if they could get to the right destinations. Like all pioneers, the Russians pushing westward are younger, harder-working, more adventurous, and more flexible than those staying behind. On reflection, the solution is both obvious and breathtaking. This new wave of anti-communist, pro-American refugees should be welcomed by millions to the world champion of peaceful ethnic assimilation: the United States.