1879, Germany invaded Sweden / Scandinavia, toppled king Johan III and made it a satellite republic, fearing the Socialist threat. (In the elections, a coalition of Social Democrats, Socialists and left republicans had gained the majority.) The British Socialists decided to interfere, occupied the nearby Färöer islands, and Iceland. Still, the Socialist union was too dominated by England / Britain to make it a truly supranational organisation.
Forming Of The Block
French uprisings and the intervention of the Red Piraters
It would take until World War I for Socialism to sufficiently expand beyond Britain. Britain stayed neutral, only providing for a little scandal by selling arms to desperate Roma Nuovo facing its defeat against Germany, though it didn't make much of a difference in the end.
Victorious Germany however failed to consolidate after the war. France, scattered for two decades and wishing reunification after the sufferings of the war, experienced massive uprisings, and French Socialists in British exile since the 1890s lobbied successfully in London to take the chance of liberating France for the red cause. During the course of 1916, France indeed got liberated and gave ground its "everlasting alliance" with Britain.
Uprisings in Germany and the North Sea turning Red
In 1918, the British-French alliance took advantage of the various workers' uprisings in (Old) Germany's industrial heartlands staged by returning soldiers who often saw themselves getting unemployed as their mines have been closed. The low morale of Germany's armed forces, especially its navy, against the overwhelming Reds also contributes to the loss of Northwestern Germany besides the loss of the left bank of the Rhine. Contributions from Atlantis just prove futile when the Scandinavians turn against their puppet governments (installed thirty years before the counter Socialism) and face support from the Reds. After the Reds stripped Germany from its access to the North Sea, they offered peace to Germany if retreating from Scandinavia which they soon accepted by the end of the year.
Red internal strife and the formalization of the Block
The reasons for the Reds to offer peace were internal affairs about the balance of power inside the expanded Socialist world. While there was a more or less widespread support for the change in government and there wasn't a hegemonial power among the Reds which could be compared to OTL Soviet Union, the French quickly became too powerful in the eyes of many non-French. So while on the one side, the newly Socialist states saw their need for cooperation if they want to consolidate internally and externally, the formation of the Socialist Block came with many compromises to be dealt with among their members.
From scratch, the Socialist Block became a highly integrated union with a common foreign policy, common armed forces, the drop of inner borders among the members and a monetary union with the new European pound currency. The headquarters were set up in Brussels, being directly on the frontier between the Romanic-speaking peoples on the on side (i. e. the French) and the Germanic-speaking people on the other side (i. e. anyone else at least then), and of course not in revolution-exporting Britain, being acceptable to all sides.
During the first half of the 1920s, the Block, consisting of Britain, Ireland, France, the Netherlands, the red portions of Germany (NW Germany the left Rhine bank), Denmark, Norway and Sweden, was primarily concerned with themselves. The next intervention were to start in 1926, when Catalans and Basques rise up against the Castillain monarchy. After Catalonia and the Basque lands are conquered in 1927, they march to conquer the rest of Spain, aided by volunteers from Northwestern Europe. Castille declines German help offers due to the king's personal hatred to the Germans. In 1928, Portugal also declares war over Spain, hoped to annex Galicia. By 1929 however, Portugal like Castille are conquered by the Socialists, leaving only Andalusia free from Socialist forces due to German support.
The 1930s prove to be relatively calm. Socialist Iberia gets reorganized along federal lines and its leftist coalition government is also quite diverse. The mid-1930s see the construction of the Channel Tunnel and a non-aggression pact with the Italians. In the late 1930s, German-supported uprisings bind the government's resources to act and keep them from further intervention for some time.
After World War II
Post-war Italy turning Red
The Socialist Block remained neutral during the whole war. But just as in the war before, the Block intervenes in countries where workers revolt, this time in Italy. After the nukage of Verona in 1944, Italy fell quickly. Italians hated the Germans, but they also hated the padrones, fascisti, the Church and the rest of the rabble that clearly seemed to fail at the war and the country. In 1945, workers rose up in Italy, and Germany was busy with defeating and disarming what remained of Russia and had therefore no means to intervene against the Reds when they came in the same year. By 1946, Socialists are firmly establied in Italy joining the Block in the same years, though anti-Socialist resistance shall continue into the 1950s. Bad harvests also cause further problems in the cold winters of the late 1940s, the Red Pirates now also being in charge for smuggling food.
Occasional foreign political backstabbing
Block agents became infamous for killing the Brasealian royal family in 1956, turning the country in chaos. In the 1970s, attempts from Socialists to seize power fail in Morocco and, eventually, in Kurdistan as well, due to German and in the latter case also Judean intervention. However, they manage to take power in Greece in 1974 and join the Block, though poor Greece rather constitutes a drain to the Block rather than a real contribution, beyond ideology, of course.
Technological achievements and setbacks
The Socialists detonate their first nuke in Lappland in 1947, thanks to successful espionage. They also launch mankind first artifical satellite in 1952. At the same year, the Block experiences a notable accident in the peaceful utilisation of nuclear energy with the core meltdown of a Norwegian nuclear power plant. Things get improved, but nobody, inside or outside the Block who's in charge, calls nuclear fission power into question.
In 1962, the Block launches the first space station, though in the race to the moon, they fall back behind both Germany and China succeeding in 1962 and 1964 respectively, they only get there in 1972. By 1980, they launched Socialism, a permanent space station, five years after the Germans and Chinese did so.
In the 1980s, the Block was leading in military technology. Milanese researchers devoloped artificial insects, the so-called "Zanzara", in the early 1980s, which essentially functions as spy drones. It took some years until the Germans follow suit. And like any other major power, they did nanotechnological experiments.