"...despite being one of the world's largest industrial economies, and with a per-capita income that nearly matched Britain, the average Belgian, particularly in Wallonia, experienced little of this, and that was before one took into account the severe ethnic and linguistic divide that separated Walloons from Flemings. The dramatic rise in the Belgian standard of living that had begun in the mid-1890s and continued through to the upheavals of 1912 had entirely stalled out and Belgian politics were polarized, angry and more than a little defeatist. The general strike of 1915 thus occurred in a context in which the Belgian working class splintered into multiple feuding factions, all holding the bourgeoisie in contempt but none quite capable of leaving their corners to compromise, a situation which of course suited the famously thuggish Leopold III just fine.
What separated 1915 from the violence of previous Belgian general uprisings such as 1883 or 1890 [1] was the thought and coordination put into it by union leaders, and that their syndicalist flavor showed for the first time. Like 1890, some observers became convinced that the country was about to tip into civil war, and that was in part thanks to the coordination between various bodies and the emergence of the Sindicat-Generale Belgique, or Belgian General Syndicate, a term of description that would become increasingly familiar to Europeans over the next quarter-century. The first of the feared "general syndicates," the SGB sought to unite all Belgian unions under one umbrella as a single industrial laborers lobby, that could take comprehensive and collective action together. This was the step beyond mere industrial unionism that morphed into syndicalism, and it was an idea that was met with some trepidation by the strike's otherwise leading light, Jules Destree. Destree was a social democrat of deep experience who had emerged from the 1890 upheavals with a passionate defense of the aggrieved laborer and was by 1915 the chief figure of the Belgian left, leading the small Social Democratic Labor Party in the Belgian Parliament and whose essays were begrudgingly read and admired by the King himself.
Destree's leftism was of a considerably more reformist school, and he had endeared himself to some moderate Flemings in 1912 when he had written the polemic On the Division of Belgium, in which he famously declared, "C'est nes pas Belgiques" - there are no Belgians. What he had meant, and with which a great many [2] on both sides of Belgium's ethnic divide agreed, was that there was no single Belgian identity and thus no way to create genuine Belgian patriotism. Destree had argued in favor of a federal solution to Belgium, with a relatively powerless central government in Brussels with powerful provincial legislatures in Wallonia and Flanders, and his acknowledgement of very real and tangible grievances of the northern half of the country had earned the attention even of Flemish nationalists like August Borms, the other major figure who emerged out of the 1915 general strike. Of course, Destree perhaps proved his point too effectively; his reform streak barely papered over his own considerable Walloon sympathies, and part of the concern expressed in his C'est nes pas Belgiques sentiment was that the larger, more populous Flanders would come in time to politically dominate the smaller, less densely populated but more industrialized and wealthier Wallonia and exercise its long-simmering grievances upon that population.
These grievances were by 1915 difficult to ignore, and the ruling Catholic Party - which managed to survive political gravity regardless of what occurred in the country - failed to address the issues head on, buttressed by an electoral system that despite enjoying near-universal suffrage had provinces alternate elections to the parliament every two years so that the entire kingdom did not go to the polls simultaneously. The sense in Belgium for both socialists and liberals that electoral politics produced little but frustration, particularly after 1912, meant that leaders like Destree or Paul Hymans, a progressive liberal, were forced to agitate from the outside, a lesson the SGB learned quite quickly.
Destree was, ironically enough for an episode that greatly burnished his credibility in Belgian politics, opposed to a full general strike and had advocated that major unions not join the SGB. Nonetheless, after railroad workers were refused a requested increase in pay that approximated to an average of about 2% per year for the next three years depending on role or profession, the rail union CCB voted to strike at the end of November, just as harvest was wrapping up and winter was approaching, and they were shortly thereafter joined by coal miners (the critical one, considering the reliance of Belgium on coal for heating, meaning that coal could not be mined or transported), textile workers, and steelmakers. The four largest unions in Belgium walked off the job at noon on November 28, 1915, plunging the country into crisis.
Leopold's reaction was, as always, quite poor, egged on by his sons - in particular Stephane Clement, a vain, vapid and cruel figure despised by the other courts of Europe who had just returned from an informal exile of a few years observing the Great American War on the Confederate lines - to crush the strikes with the army. The problem came when several soldiers, particularly Walloons, refused to open fire on strikers, and many instead crossed over to join them on their lines. Strikers suddenly had rifles and ammunition and were barricading themselves in scenes reminiscent of Paris in 1832 or 1848, and socialist leader Emile Vanderwelde called for a "Brussels Commune" as he returned from exile in Britain to Ghent on the eve of the strike. By December 2, the country looked ready to explode, with more soldiers called up from barracks, socialist paramilitaries patrolling much of Liege, Charleroi and Ghent, and the King nervously studying evacuation plans from Brussels with the path to his preferred destination, France, largely blocked by the uprisings.
Destree, in a fiery speech in Parliament, called on the government to "hear at last the people's voices!" to which Flemish parliamentarian Borms, who had up until then been an obscure if outspoken professor of art and language in Antwerp [3] responded with a shout "Which people? Whose voices? In which tongue?" Destree smiled and shrugged it off, but Flemish parliamentarians, many in the governing Catholic Party or the opposition Liberals, roared in approval, and Borms the next day, on the 4th, gave his own speech, largely off the cuff, in which he accused the socialist movement of being a "front for Walloon influence" and then pilloried his own Catholic Party for ignoring its Flemish voter base to instead act as a catspaw for "French influences in the Belgian state."
By the time the strike ended on December 10 with sixteen dead, multiple riots put down by force but also the strikers largely getting what they wanted - thus teaching both the government and the SGB that their approach had worked and that they would need to be even more violent and demanding next time - Belgium would never be the same. Vanderwelde went to prison, martyring him in the eyes of syndicalists but also opening the space for Destree to become even more the centrifugal figure of opposition to the tired and clientelist Catholic Party, and the voice of grievance of the Belgian worker had been breathed into existence and underpinned his political status. But simultaneously, Flemish nationalism for once had its own champion rather than just idle grumblings, and though the Flemish right would be divided for years to come - a great many nationalists were liberals uncomfortable with Borms' proto-integralist worldview - it had finally started to find its own voice..."
- Path of Darkness: Europe's Illiberal Hour
[1] Belgique Rouge was a while ago, and I did briefly consider using that for the update here, so I believe these were the years that Belgium last experienced major strife.
[2] Including IOTL King Albert of Belgium, though Albert disagreed with his conclusions in calling for a federal Belgian state
[3] Right-wing weirdo-intellectual professors and mass politics in early 20th century Europe, name a more iconic duo