This is a much skimpier answer than I like giving to a good question – in fact, I won’t be offended at all if the mods want to purge it – but annoyingly this is one of those subjects best covered by physical resources (you know, books) I simply don’t have access to right now. I can at least give a brief outline now, but more detail and follow-ups would need to wait until I can go visit a decent library again.

An anti-Franco resistance did operate for some years after the end of the war, most famously depicted in the film Pan’s Labyrinth (in the scenes without mythical creatures, at least). Sadly, the film probably exaggerated the successes of these guerrilla groups in defying the new Francoist regime. Several factors undermined the immediate success of resistance groups. For one, the war itself had been long and exhausting, and even many who favoured the Republic must have balked at trying to continue the struggle any further, particularly given the noted deterioration of living conditions in Republican territory over the course of the war. Moreover, the Francoist state – which had already honed its repressive methods in conquered Republican territory during the war – clamped down effectively on any political or military resistance, embarking on a wave of post-war punishment and repression targeting Republican supporters.

Perhaps most interestingly, however, many dedicated Republicans had fled the country entirely rather than taking to the hills, most commonly ending up in makeshift camps in southern France. While many had returned to Spain in the months after the war’s end after Franco promised them amnesty, most of those who remained were among the Republic’s most fervent supporters, who (likely rightly) expected that their returning to Spain would not be good for their health. The result was that southern France was home to significant population of anti-fascist Spanish Republicans when France was defeated in mid-1940. Many of these Republicans escaped the camps, and went on to form the heart of the French Resistance in the region – most estimates I’ve seen put their numbers at about 10,000. In other words, there was a larger, more effective Spanish resistance against Germany in France than there was against Franco in Spain.

These Spanish resistance fighters were, of course, no friends of Franco, and understood their struggle in France to be part of the same broader anti-fascist war they had been fighting previously. They did not expect that Franco could survive an Axis defeat, and were awaiting their opportunity to return to Spain to renew their original struggle. Following the successful Allied invasion of France, one faction of the Spanish resistance in France decided that the time was ripe in October 1944, crossing the Pyrenees with a force made up of thousands of experienced and well-armed guerrillas, aiming to conquer part of the border, and spark a broader revolt against Franco in the rest of the country. Needless to say, this did not turn out too well – the wider revolt never materialised, and Franco was able to respond with overwhelming force, driving the guerrillas back over the border within ten days.

There is a great deal of ugly political factionalism and intrigue – particular among Spanish communists – that I’m skipping over here, because I simply don’t know the history well enough to recount it without referring to the literature. While attempts at armed resistance continued after the failure of the October 1944 invasion, the numbers involved were smaller and their efforts posed relatively little threat to the regime (which is not to say that they weren’t vigorously persecuted and hunted down). As we know, Franco managed to survive the defeat of the Axis, and remained diplomatically isolated yet in power at the end of the Second World War, and with the onset of the Cold War, hopes for Allied intervention in Spain dwindled away. In 1948, the Spanish Communist Party decided to stop supporting armed resistance, and by the early 1950s the remaining guerrilla groups had either been eliminated, surrendered or fled the country. This was hardly the end of political resistance to Franco within Spain – and, in a certain light, the actions of groups such as ETA might be considered a resumption of armed resistance to the regime – but marked a shift away from strategies which emphasised the military defeat of Francosim.

The book I’m missing most here is Jorge Marco’s Guerrilleros and Neighbours in Arms: Identities and Cultures of the Anti-facist Resistance in Spain (Brighton, 2016) – there’s a substantial literature in Spanish I’m a bit less familiar with, but this is the best study available in English.