The scission between revolutionary parties in Europe happened because of World War I. Beforehand, there was a plethora of different movements, views and theories, mostly inspired by Marx, within the Second International. The advent of World War I created a deep divide between these political movements; the left wing opted for an anti-war stance, whereas the moderate wing put forward its "patriotic" duties, asking to support their national governments at war. Therefore, instead of a universal, immense socialist system, the moderate wing opted for domestic reform, forming "social democracy." On the other hand, Lenin created the Third International, consolidating the divide between revolutionary (communist) parties and the moderate wing.
Social democratic parties were mostly successful in Western European countries in the aftermath of World War II.
The Scandinavian example has been very attractive for most social democratic parties in the world. It is, however, worth underlining that the 30 glorious years, as the period between 1947 and 1973 are called have also seen examples of good governance on the part of right conservative parties. German miracle and recovery owes much to Ludwig Erhard and Italian recovery has been done through the guidance of De Gasperi's Christian Democracy.
Revolutionary communist parties have become less revolutionary and more conventional under a USSR-type of socialism and have lost their attraction in Europe, except in the southern countries, Italy, France, Portugal, Spain and Greece, where the organization and popular basis of these parties remained strong, even under fascistic regimes of Franco, Salazar or Papadopoulos. On the other hand, Social Democracy thrived, remaining chiefly a domestic ideology based on distributive justice, without challenging the essence of capitalism. In most European countries, the alternation in government turned into a bipartisan issue between center or conservative right and socialist/social democratic movements.
The last wave of elections in Europe, not least the European Parliament (EP) elections, have shown us that such domination by the center-right and center-left parties is mostly over. Not that their importance has totally vanished, but traditional center-right and center-left parties will no longer have a majority in the EP, and this is a first.
A lot has been written about the rise of the populist/fascistic extreme-right on one side and of the Green parties on the other. As wisely underlined by Sheri Berman in Social Europe, political scientists call such parties "niche" parties, meaning that they gather votes around a single, hot issue. For the populist right, it is immigration and the fear created by globalization. For the Green parties, environmental issues starting with climate change remain the core issue.
Still, such developments in votes do not show similarities from one country to the other. Finland is one of the EU states with the least number of refugees and migrants, still the extreme-right has become extremely powerful; whereas in Spain, a major destination for illegal immigration, the extreme-right remains anecdotal.
While Labour is in deep trouble in England, the Socialist Party is evaporating in France and the SPD is in shambles in Germany; Portugal, Spain and Denmark give strong support to their own social democratic movements.
The more social democracy, which is a national ideology not very fond of European integration refuses to change, the more disaffection of the popular masses will be visible. The situation of the Labour Party in the Brexit process is a good example of such prevarication. It is high time for socialist parties to uphold the values of the European Union, in a sincere and constructive way, to really redefine themselves. The Green parties, for the time being, are not really an alternative to social democracy, and the liberal center remains what it is, a union of liberal views. It is also worth remembering that the only political forces in Europe (in spite of visible differences from one country to the other) to support an "EU member Turkey" remain with the Green parties and social democratic movements.