IN the wake of communism's collapse and the reawakening of old ethnic rivalries in eastern Europe, it has become common to speak of similarities between the current situation and that in the early part of this century.So it was challenging to this mind-set to be reminded by a senior German journalist, Lothar Ruhl of Die Welt, that Europe is in fact back to the position it enjoyed in about 1600. Mr. Ruhl's point is that Europe is again in the process of becoming a single unit. And, although he did not say so explicitly, this unity comprises essentially the area defined by Western Christendom. Poland remains the frontier, with Russia not directly included in Europe. And this particular delineation of borders includes virtually all those countries most likely to become associated with the European Community (EC) over the next generation. The present debate over enlargement of the EC often uses the vocabulary of "widening" versus "deepening." The Germans I have spoken with in recent weeks believe that both must happen simultaneously. That is, the institutional mechanism of the EC must be deepened, but this deepening cannot be at the expense of applicant countries waiting longer than they otherwise would to become members. By January 1993, the economic union should be complete. As of that moment, only 15 months away, there is supposed to be complete mobility of the labor force within the 12 countries. Understanding the challenge that even this development presents makes one appreciate the dilemma the nations of Europe are currently facing over immigration policy from without. There is also strong support for the economic union becoming some kind of political union, although the implications of this are far from spelled out. In the immediate future, the challenge is to complete the monetary union, a step that Britain is widely blamed for holding back on. On the other hand, Norbert Walter, economist for the Deutsche Bank, cautioned that over the next few years it may appear that Germany itself is trying to slow down the move to monetary union. Actually, he claimed, this would only be a sign that the Germans want the moves to be correct ones, "in line with what the Bundesbank would do." Does this expressed attitude, as well as the fact that 80 million Germans sit in the middle of Europe, mean that Germany is destined to dominate the emerging market? Not necessarily. What also matters is the intellectual contribution the other main members of the EC bring to its development. Germany will almost surely benefit, though, from its historic trading ties to eastern Europe. And, if the nations of the Soviet Union get started on a free market basis, the products of German industry will be wanted there as well. Most observers in Europe expect the community's enlargement to come in three or four distinct stages, over as much as the next two decades: first, Austria, Sweden, and Norway; then, by the end of the century, Poland, Czechoslovakia, and Hungary; somewhat later, the three Baltic nations; and finally, whatever independent nations emerge from the present fighting in Yugoslavia. This map indeed approaches the Europe which existed as a single cultural unit at the end of the Middle Ages. Russia is generally excluded from this scenario on the grounds that its potentially huge economy needs to be the centerpiece of another major trading bloc. Besides immigration policy, which is of first concern to several of the EC countries today, the economic union poses questions for their educational systems. Some Germans complain that their university system, often taking five or six years to complete, places young graduates on the job market as much as three years later than their French counterparts. It is, of course, still to be seen how mobile the European labor force becomes. But with legally guaranteed mobility, even marginal shifts in the labor force may have major impacts on such heretofore domestic institutions as a country's university system. All in all, this is a heady moment for European development. Progress toward some kind of federation will almost surely continue, but one should not be surprised if, along the way, there are periods of questioning or even momentary reversal.