It depends on when.
When the Spanish Civil War broke out in July 1936, the French Popular Front government of Leon Blum had been in power less than six weeks. Asked by the Spanish Popular Front Coalition to send military aid, Blum had considered it, but was told by the G.Q.G. that if they did so they ran the risk of a rebellion in the French army. Given the chronic instability of governments in the French Third Republic, any rebellion in the army, no matter how small, would have resulted in the collapse of the government. As it was, just the rumour that Blum’s government was going to supply bomber aircraft to the Spanish Republicans was enough to nearly split his government; the proposal to send direct military aid would have been certain to. The election of the French Popular Front in June had seen industrial action on a massive scale, with unions occupying factories and industrial sabotage; with the attempted coup in Spain followed by civil war and rumours sweeping France of a similar army uprising, it wouldn’t have taken much of a trigger for the trade unions to form militias in response to what they saw as a threat to the republic. Blum would have been faced with civil war to his south-west, a recently reoccupied Rhineland to his east, a bellicose Italy to his south and potential anarchy at home; treacherous water to try navigating, particularly for a new and not particularly popular Prime Minister. And it would have been a situation the French would have had to deal with alone; the British Baldwin government saw the Spanish Civil War as potentially the trigger for another World War and were determined to avoid involvement under any circumstances; not even German warships firing on British flagged freighters as they approached harbours in the Basque provinces, or the attempted torpedoing of a British destroyer by an Italian submarine, resulted in any reaction from the British.
A later intervention, considered purely out of self-interest and as self-defence is more likely, in other words not to support the Spanish Republic but to defend the frontiers of France; in late 1938, when the fighting in Spain entered Catalonia, French Prime Minister Daladier did consider sending the army into Spain to create a buffer zone between the Nationalists and the French border. Before doing so he consulted the British regarding how they would respond to such an action. Halifax informed the French that, as far as Britain was concerned, it would be considered a direct provocation towards the Fascist powers and would nullify British obligations towards the Treaty of Locarno; if Germany and Italy responded by attacking France the British would not come to France’s aid, the French would be faced with fighting Germany, Italy and Nationalist Spain alone. This was in the aftermath of the Anglo-French betrayal of Czechoslovakia at Munich so there were no doubts on the French side that the British meant what they said. Daladier wasn’t the type to take such a risk, but there were men in Daladier’s cabinet that probably would have, Georges Mandel and Paul Reynaud to name the two most likely. Mandel was rock solid, but his Jewish ancestry worked against him just as it had against Blum, so an early Reynaud premiership is the most likely.
A direct French intervention to aid Spain under a Reynaud government is close to impossible, the League of Nations had already quarantined Spain in the form of the Non-Intervention Pact and France was one of the signatories. But a Catalonia that has seceded from the Republican government and appealed for international recognition prior to the Nationalist forces approaching their territory might have stood a chance; the French could have argued that the Catalan people was exercising their right to Self-determination, as enshrined in the League of Nations’ charter and that the French were not invading, but were responding to a plea for help from a people who needed protection prior to taking their rightful place in the community of nations. Done quickly enough and they might have been able to secure Catalonia while Franco was still dealing with the remainder of Republican Spain. Done very quickly and they could have presented Europe with a fait accompli.
The problem is that the French army didn’t do ‘quick’. General Gamelin probably would have wanted to mobilise the entire French army before doing anything, given the circumstances a large mobilisation of French reserves would have been prudent anyway but Gamelin would have insisted on calling up all France’s reserves before a single soldier stepped over the line into northern Spain. Most of France’s huge army would not be facing Spain; instead they’d be manning the fortifications of the Maginot Line, the lesser fortifications of the Italian frontier, and in positions along the Belgian frontier, all there to defend France in case Hitler and Mussolini responded as Chamberlain feared they might and attacked France directly. With the remainder Gamelin would occupy Catalonia. This wouldn’t be a lightning swift display of audacity like Hitler had stunned Europe with when he’d occupied the Rhineland, Austria and Sudetenland, instead Gamelin would have advanced very slowly and cautiously into north-east Spain, digging in every step of the way; that was simply Gamelin’s view of how advancing into hostile territory was conducted.
In World War Two, Reynaud had repeatedly tried to have Gamelin dismissed but hadn’t been able to because Gamelin was Daladier’s man and Reynaud had needed Daladier’s political support to survive. This had come to a head late the 8th of May 1940 when, frustrated for the last time, Reynaud announced to his cabinet colleagues that he could no longer serve as Prime Minister and submitted his letter of resignation to President Lebrun on the 9th; less than twelve hours later the Germans invaded, forcing Lebrun to tear up Reynaud’s resignation and Reynaud to put up with Gamelin for two more weeks. So a French campaign that was seen as very poorly handled could see either the end of General Gamelin or the resignation of Prime Minister Reynaud. Since this scenario requires Daladier to have lost the premiership early, perhaps he would not have been so quickly politically rehabilitated and wouldn’t have been able to defend Gamelin and Reynaud would be able to replace him without triggering the collapse of the French government. If that were the case, the command of the French army would most likely fall to General Georges; he’d owed his rise in the French army to his professional capabilities, not to political connections and had been expected to replace Weygand in 1935 but Daladier had considered him politically unreliable.
If Georges replaces Gamelin, there is an opportunity for some limited improvements in France’s overall defences, but the depth of systemic problems in the French military would have required several years to correct, even if they had been correctly identified and the money were avaliable. Certainly Georges would have been an improvement on Gamelin and Weygand and with political backing from Reynaud could have made some significant changes. The problem though is that the French left already suspected that a professional army corps would be an armee de coup d’tat, if Georges were the head of the army and proposing it, they’d be sure it was.
For General Franco, French intervention would have been infuriating; by 1938 Republican defeat was seen as inevitable and Franco had actually been taking his time in completing his victory so that he could use the fog of war to conceal the massacres of his opponents in newly occupied areas. Just when he’d be turning looking to the final campaign to wind-up the war, in step the French to take one of his richest regions. And there wouldn’t be anything he could do about it except shout in rage from the balcony of the presidential palace in Madrid; the entire Nationalist army could never have challenged even a single French army and his international backers would not have been interested in taking risks for his sake.
Juan Negrin, Spain’s Republican Prime Minister would have considered it the final betrayal, as would the French Communists and the Comintern. Negrin’s strategy throughout much of his premiership was to try to involve the democracies in Spain’s war, or for the Republic to hold out long enough to be saved by the outbreak of a wider European war, which he saw as inevitable; if the French army had crossed the border to cut away his most important and powerful power base, it would have been truly ironic.
For Stalin, in wake of his exclusion from the Munich Conference and subsequent carve up of that Soviet ally, this new French betrayal of another Soviet ally would have confirmed his deepest suspicions; the Capitalists were carving up Europe for their own ends and they would soon turn their sights on the Soviet Union. How could he trust a supposedly socialist French government that betrays one socialist nation after another? He wouldn’t have even bother paying lip service to any further negotiations for an alliance.
For Mussolini, French intervention late in the war would have been a mixed blessing; he’d have had an excuse to keep the air and sea bases in the Balearic Islands that gave him the strategic reach into the Western Mediterranean that he so coveted but would have come at the price of more French forces in the region.
For Chamberlain it would have been a reckless provocation, unnecessarily endangering Europe with another war just twenty years to the day when the guns had fallen silent on the First World War. It would have many Englishmen wondering just who was more likely to start another world war, the Germans or the French, and to either find more forceful ways to curb Gaelic recklessness, or put a bit of political and diplomatic distance between them so as not to be drawn into the inevitable maelstrom with them. Certainly any territorial guarantees in Eastern Europe, in a region that Britain did not have direct interests in, to aid a French ally, would be extremely unlikely since it would only further embolden the French and lead to even more reckless behaviour.
For Hitler it would have been an absolute godsend; just when it looked like the British and French might be moving closer together and resolving to frustrate his ambitions, here would be a wedge that would drive them further apart. It would have also been golden propaganda material; who could object to him making Bohemia and Moravia German protectorates when Catalonia had just been made into a French protectorate. Other than lending some moral support to Franco he’d have done nothing directly, would have ordered the Condor Legion to remain well clear of French forces and not to engage French fighter aircraft and if Franco wanted to press the issue Hitler would have threatened to withdraw German support; Germany in late 1938 – early 1939 was still three to four years away from being ready to fight a major war and Hitler new it. Instead he’d have accelerated the absorption of the rump Czechoslovakia and considered what new opportunities this division between Britain and France offered.