Adapted from an earlier answer:

You’ve asked a bit of a chicken and egg question. There were, of course, parts of the world where fascists didn’t come to power in the interwar period, and parts where they did. But did they fail to achieve power in some places because they were opposed by people (ie anti-fascist movements), or because the fascist movements themselves were weak, or lacked the right circumstances to succeed?

I’ve addressed a narrower version of this question a couple of times in a British context, namely whether the Battle of Cable Street in October 1936 can be seen as contributing to the failure of the British Union of Fascists. I argued then, as I would now, that Cable Street was not the decisive turning point in the BUF’s fortunes. Given the continued strength and stability of Britain’s mainstream political parties, and the absence of many of the structural factors that made fascism appealing elsewhere, I don’t think the BUF would have succeeded anyway. However, the scale of the failure of the BUF can be linked to anti-fascist activism, both in terms of local opposition constraining their ability to spread their message, as well as in less obvious ways such as forcing fascists to reveal their true nature through provocations, as at the notorious Olympia Rally in 1934. Personally, I think that even if British fascism would never have succeeded, anti-fascist movements limiting their success, influence and ability to bully vulnerable populations was incredibly worthwhile.

Your question is, of course, somewhat broader than just Britain, and a useful approach for understanding what did and didn't work would be to look at contexts where the struggle was very finely balanced, where fascism came very close to succeeding or failing.

There is one very obvious candidate to discuss in the context of interwar Europe: Spain.

Spain during the first half of the 1930s was something of an anomaly in European politics. While most interwar democracies folded and were replaced by various shades of authoritarian (generally right-wing) regimes, Spain went the other way in overthrowing first the military dictator General Primo de Rivera, and then the monarchy soon after, and establishing the Second Spanish Republic. The Republic itself was founded as an explicitly progressive project, designed to create a secular, reformist state that would modernise Spain in line with European norms. The idea that the left might lose an election – as they did in 1933 – and the right could then dismantle many of these reforms was not seen as the natural consequence of the democratic process, but as undermining the Republic itself. The new right-wing government’s anti-reform policies met with substantial resistance, with widespread strikes eventually culminating in an attempted insurrection in the mining region of Asturias in 1934, which was eventually brutally crushed by the military using colonial troops. Wherever one’s sympathies lie, neither side was operating within what might be considered liberal democratic norms. The next two years saw ongoing political violence instigated by both left and right wing factions.

By this point, the left had started learning the lessons of European politics in the 1930s: division meant isolation meant defeat. This was most obviously the case in Germany, where mutual antagonism between socialists and communists prevented a common front against the rise of Hitler, who was able to swiftly demolish both groupings after gaining power. One solution proposed was known as the ‘Popular Front’ – a broad coalition of anti-fascist parties, from liberals to communists, who would agree to work together on a common platform and defend the status quo (ie liberal democracy) against the right. For groups such as the communists, this meant temporarily abandoning attempts to instigate a revolution. Such an approach was particularly attractive in Spain, as the Republican electoral system greatly favoured pre-election coalition agreements.The election victory of the Spanish Popular Front in February 1936 was in a real sense an anti-fascist victory – even though Spain had a relatively small fascist movement (the Falange), the anti-democratic right was still a significant and growing force. Typically for the Republican electoral system, a relatively small margin of victory in terms of votes had led to a massive parliamentary majority for the left, with the socialist party and liberal Republicans the main beneficiaries in terms of seats. Right-wing political and military leaders began plotting to overthrow the government soon after the Popular Front victory, but it was not until mid-1936 that plotters managed to make decisive headway in developing support in the armed forces, many of whom were reluctant to act against the government absent a revolutionary threat to the established order. However, the murder of José Calvo Sotelo, a prominent right-wing politician, itself in retaliation for the assassination of a socialist-aligned officer in the Assault Guards (a paramilitary police force, the urban equivalent of the Civil Guard), was the catalyst that convinced a substantial body of opinion in the military that Spain was descending into chaos, requiring the military to step in and restore order.

The resulting military coup, however, was unsuccessful in toppling the government. In most of Spain’s major cities, it failed thanks to a combination of loyalist elements of the police and armed forces, and popular resistance from left-wing political parties and trades unions. However, much of the armed forces were won over to the rebellion, including the Army of Africa in Spanish Morocco (the only part of the Spanish Army that actually knew what it was doing). Crucially, the rebellion received immediate support from Italy and Germany, initially providing vital logistical support (such as helping transport the Army of Africa to the mainland), and eventually committing large numbers of troops to help the rebellion directly. While the Republic received (somewhat less) support from the Soviet Union, Western democracies adopted a policy of ‘Non-Intervention’, seeking to limit the conflict’s scale by preventing the export of war materiel. Given that fascist powers (and the USSR) were willing to sign the agreement and then completely ignore, it was rather ineffective.

Even without this international interference from fascist powers, the ensuing Spanish Civil War would have almost certainly taken on an anti-fascist character. The underlying logic of the Popular Front was anti-fascist, and the Spanish left were primed to view their struggle against the authoritarian right within this wider European context of the advance of fascism and the failure of democracy. Much of the world agreed. Whatever the local complexities, Spain seemed to readily reflect the much wider ideological conflicts of the 1930s. It was easy to read European politics into the situation – here was a democratic government under attack from a right-wing, (quasi-)fascist uprising, directly supported by Hitler and Mussolini. Crucially, the resistance to the coup attempt and then against international intervention seemed to be the first moment anywhere that it seemed possible fascism could be defeated - while democracy had died with a whimper throughout Europe, Spain seemed to prove that anti-fascist unity could actually work. As such, the Spanish Civil War became a major cause for the anti-fascist left across the world, the flashpoint of a much wider ideological struggle, similarly to how the Vietnam War became emblematic of Cold War conflicts. Solidarity movements abroad grew to a scale never really seen before, raising huge amounts of money to provide support and humanitarian relief for the Republic. Large numbers of foreigners went to Spain to fight for the Republic, approximately 35,000 all told. This remains an unprecedented mobilisation of foreign volunteers for a single conflict, and these volunteers represented a crucial tie between anti-fascist activism in their home societies and the Spanish Republic.

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