POPPING out of the tops of lush green trees where the Hungarian forest meets Austrian wheat fields in Burgenland state, an old Hungarian border tower stands as a solitary reminder of the way things used to be."Now we're the ones with the guard towers," says Gunther Hauer, a fresh recruit to the Austrian police, as he points out the towers recently built by the Austrian Army assigned to reinforce border surveillance. "Two years ago, it was the Hungarian Army's job to keep people in," he says, "and now here we are doing our best to keep people out." The fall of the Iron Curtain has opened a new front in Western Europe's battle with immigration - traditionally oriented toward the south. After a sharp rise in entries from the East following the demilitarization of borders between Western and Eastern Europe in late 1989 and early 1990, the number of people attempting a move from East to West has fallen off. Some experts now say fears of massive migrations were never warranted. Yet even if illegal entries of Hungarians or Czechoslovaks have dwindled, the number of Romanians caught here in Burgenland state remains high. And sophisticated underground networks for getting illegally entering Poles to jobs in the West are frequently discovered, as in recent cases in Paris. Increasingly, people from developing nations also are attempting entry to the West by way of Europe's so-called "green border." A recent log of apprehensions in Austria's Eisenstadt district, where Mr. Hauer works, includes a Chinese, a Mongolian, five Pakistanis, three Nigerians, and a Sri Lankan, in addition to the majority Romanians. The number apprehended in this one district often tops 20 a night. "We are getting more and more people from the third world, people with no documents and no legal status. It's a real security problem, and it is only going to get worse," says Gerhard Wild, assistant director for the Burgenland district police charged with immigration affairs. Still, what has some officials most concerned is the opening up of Soviet borders in 1993, when the recently voted "freedom of movement" takes effect. The prospect of what Soviet officials predict might be millions of jobless Soviet citizens moving West is perhaps most unsettling to Eastern European officials, who once again see themselves serving in an East-West battle zone - this time with people, not arms, at issue. "It's the future of the Soviet Union that has us terrified," says Andras Rakovszky, Hungarian ambassador to the Council of Europe in Strasbourg, France. "Even if we serve as no more than a path for transit, it will be overwhelming for us." SUCH concerns are at the root of Western interest in massive aid programs to help the Soviet Union avoid economic breakdown. The topic dominated the summit of the Group of Seven industrialized nations in London last month. Some Western officials have even accused the Soviets of using those fears to blackmail the West into providing huge amounts of assistance. Others, however, do not believe a mass Soviet emigration will materialize. "I remain optimistic about the Soviet Union, I don't believe we will see millions of Soviets coming to the West," says Erich Kussbach, who heads the legal and consular department of the Austrian Foreign Ministry. Immigration specialists who agree say a number of factors will work as a brake on large-scale departures: the Soviet Union's poor travel infrastructure, the inability to convert rubles to foreign currency, visa requirements throughout Europe for Soviet citizens, and tightening border controls. Dr. Kussbach, who helped organize the first-ever conference on immigration including both Western and Eastern European officials, says, "Our biggest advantage is that we have a year-and-a-half to work with the Soviets on this." Programs already being set up with Eastern European countries to encourage cooperation on immigration issues will have to be extended to the Soviet Union, Kussbach and other West European officials say. "What are needed are well-structured plans that acknowledge that immigration from the East and eventually from the Soviet Union is going to exist, but that planning and coordination among countries can reduce the level and the hardships for everyone involved," says Candido Cunha, a specialist on immigration legal issues at the Council of Europe. SHORT-TERM training courses are already being organized for East Europeans, in both the East and the West, to help workers attain the skills needed to find employment at home. Better information on the reality of conditions in the West is also being diffused. Austria has a pilot program for inviting Polish journalists to Austria to report on employment prospects and housing shortages. Following their immigration conference in Vienna in January, European countries have also set up working groups to coordinate policies in such areas as border controls, visas, political refugees, information exchange, and labor requirements. Of course, programs alone are not going to solve what is essentially an economic question. "As long as it is possible to earn here in three days [what it takes to] live in Czechoslovakia for a month, we are going to have these problems," says Ingrid Nowotny, a labor migration specialist with the Austrian Ministry of Labor and Social Affairs. Still, many European officials express optimism that Eastern Europe's economic transition, and thus the bulk of East-West immigration (excluding, perhaps, the Soviet Union), will be "solved" over a relatively short period. "There is a general feeling that, given Eastern Europe's industrial tradition, they will be successful at finessing their own redevelopment, that out-migration will eventually stop, and that consequently this is a temporary situation," says Mr. Cunha. Some observers believe, however, that the focus on the East will only tend to put off addressing the continuing and, in fact, building pressure for immigration from the South, especially Africa, and that has many experts worried. "Unquestionably we have to address the issue of immigration from Eastern Europe," says Mateo Sorinas, secretary to the Committee on Migration, Refugees, and Demography of the Council of Europe's parliamentary assembly. But given the South's high population growth, prospects for political turmoil and economic calamity, and a diversity of cultures foreign to that of Western Europe, Mr. Sorinas and other experts say it is third-world immigration that presents greater challenges to Europe. Sorinas say: "We risk solving the part of the problem that presents the fewest long-term hazards."