None of us were around for 1789 or 1848, but at least some of us had the good fortune to witness 1989, that unforgettable year of revolutions.

Germany overnight has been marking the fall of the Berlin Wall 25 years ago, the most emblematic moment of the end of the Soviet empire in eastern Europe.

It’s a moment the whole world can celebrate, but some have more cause for celebration than others. It’s worth remembering that different countries have had quite different post-Soviet fortunes, and perhaps pausing to consider why that might be.

In addition to the former East Germany, which is now simply the five eastern states of Germany, there are — depending on exactly how you count — between 28 and 33 countries in the old Soviet share of Europe (including the whole of what was the Soviet Union). Some have been striking successes: Poland, the Czech Republic, Estonia and Slovenia, for example, now sit securely within the European political and economic mainstream.

Others, however, seem trapped in the authoritarian past, including, increasingly, Russia itself. In some the old states autocrats simply made small changes in their rhetoric; others fell prey to territorial conflicts and destabilisation from outside. In just the last few months we have seen Ukraine go down that road.

Interestingly, there’s quite a good rule for working out which countries are in which group. The test is whether or not a country was already part of the Soviet Union prior to World War II.

Territory that now amounts to 17 countries (18 counting Kosovo) fell under Soviet control only during or after the war. Some (the Baltic states and Moldova) were formally incorporated into the USSR; the rest remained nominally independent. But all of this group have now succeeded in building functioning democracies.

The path was easier for some than others; the breakup of Yugoslavia led to a series of wars in the 1990s. But despite a number of continuing problems, none of the countries in this group look like sliding back into dictatorship or civil collapse. Most of them have joined the European Union, and the remaining half-dozen or so are in the queue.

Against the background of how the world looked in 1989, the reintegration of this area into Europe has been stunning.

Further east, the picture looks very different. The 11 countries (plus four semi-recognised breakaway regions) that formed the Soviet Union prior to 1939 have no such success story to tell.

Georgia is the only one whose democratic structures look in even reasonable shape; elsewhere democracy is fragile or non-existent.

Ukraine is engaged in a war with Russian-backed separatists in its east, while several others are the scenes of frozen conflicts. Things could be worse, and in Soviet times they probably were, but the high hopes of 1989 have been disappointed.

The 1939 line of control really seems to matter. Moldova’s breakaway pro-Russian region, Transnistria, is the area east of that line; conversely, Ukraine’s most stubbornly pro-European area is to the west of it (the green area on this map).

Why is this? It’s not as if the pre-war experience were anything wonderful for many of those “western” countries: Romania, Yugoslavia and Albania remained backward and autocratic. Yet they seem to have been able to draw on a sort of European consciousness that Russia and the others lack.

No doubt there’s a strategic element involved; both NATO and the EU have made a judgement about how far eastward they’re willing to extend their influence. But it’s at least a striking coincidence how well that present policy matches the historical reality.

Perhaps there’s some sort of threshold for historical memory that kicked in at about 50 or 60 years. Or maybe the Soviet experience in the 1920s and ’30s was just so traumatic that it left no basis on which to rebuild civil society — at least not without more than 25 years of effort.