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France was not opposed to imperial intervention in Corsica in principle. The French, after all, had specifically approved it in the Treaty of Fontainebleau which they had signed with Genoa and Austria in 1737. Vienna's action on this treaty had been preempted by the outbreak of war with the Ottomans, but presumably if an imperial presence on Corsica had been completely intolerable to the French their negotiators would not have agreed to the treaty in the first place. Perhaps desiring to establish Genoa and its dominions as a de facto French dependency, the French chief minister Cardinal André-Hercule de Fleury was clearly not eager to share the "pacifying mission" with the emperor, but the mere proposal of an imperial occupying force as permitted by the treaty did not pose a serious threat to French policy.

What alarmed Fleury and his fellow ministers was the prospect of British involvement. The British, of course, were not signatories of the Fontainebleau treaty and could not intervene on Corsica even if they wanted to. At that moment they had bigger issues on their plate, having declared war on Spain on October 22nd over matters of colonial trade.[A] The activity of their ministers in Vienna, however, was noted by French intelligence, and given the alliance between Austria and Britain it seemed plausible that imperial intervention could act as an opening for British influence in Corsica, precisely what France had been trying to prevent by intervening in the first place. The French consul in Livorno reported that the imperial commander there, Lieutenant-General Karl Franz von Wachtendonck, was believed to have regular conversations with both the English consul Burrington Goldsworthy and Corsican rebel agents. Rumors that Grand Duke Franz Stefan of Tuscany was also involved only strengthened their suspicions, as his clumsy attempts to influence the rebel movement for his own purposes in 1737 were well known to the French. All clues seemed to point to some sort of British-Tuscan-Corsican plot to pry the island away from Genoa and into the British sphere of influence, in which Emperor Karl VI was either an unwitting dupe or an eager accomplice.[1]

While generally opposed to the idea of new French deployments on Corsica, the Genoese knew their position was as weak diplomatically as it was militarily. The French had every right to dispose of their forces on Corsica as they wished; the Treaty of Fontainebleau, which the Genoese had signed, contained a provision that “even though the Republic should refuse these offers [of assistance]” the French and Austrians “shall nevertheless take the necessary measures to extinguish the said Rebellion.” Furthermore, even the most paranoid senators had to admit that but for French help Corsica would likely have been lost in its entirety. The Senate had to walk a thin line—if they meekly allowed the French to act as they pleased they feared a French takeover, but if they acted with too much hostility the French might simply pack up and leave, which might not only lose them Corsica but cause a diplomatic breach with France with serious consequences for the security of the state.

The only coercive power which Genoa possessed was the power of the purse. The implementation agreement with the French had required a financial obligation of two million pounds, of which 700,000 were paid up front. By the autumn of 1739, that two million figure had been met and exceeded. The Republic could not deny France the right to land troops on Corsica, but it could refuse to pay for them, forcing Paris to saddle the financial obligations for a war it was waging on Genoa’s behalf. Certainly France could pay their way, for despite the notorious problems and inefficiencies of the French fiscal system and military administration the Corsican venture was a minor affair by the standards of the mightiest state in Europe. There was a general unwillingness in the French government, however, to shoulder the whole financial responsibility for what was seen as Genoa’s fight. That Genoa should refuse to pay for what was essentially a favor from Paris seemed immensely ungrateful.

As diplomats, consuls, and senators sparred, there was a continual and ever-growing pressure on the French commander in Corsica, Lieutenant-General Louis de Frétat, Marquis de Boissieux. His government had already begun to grow weary of his strategy of containment, which seemed to produce no progress despite mounting casualties. The morale of the troops was low, diplomatic relations with Genoa were at their lowest ebb, and Theodore's rebellion did not appear to be collapsing with any rapidity. The defeat of a French battalion at the Battle of Ponte Truggia on September 17th, followed by the loss of most of Vico and Cinarca to the rebels, appeared to illustrate the exact opposite—the rebels were not only still willing to fight, but were on the offensive. All this was bad enough, but the specter of imperial intervention and fears of a British-backed plot really put the screws to the general. If he could not deliver, and deliver soon, Versailles would replace him with someone who could.

In mid-October, Boissieux had ordered a large-scale attack by Brigadier Villemur into Ampugnani and Casaconi with around 2,000 infantrymen and hussars. The attack managed to catch Lieutenant-General Count Andrea Ceccaldi quite off guard, as he had been anticipating an attack in Alesani. The local militia was crushed, and an Ampugnani native, Captain Gio Tomaso Franzini, gained the honor of being the first Knight of the Order of Deliverance to be killed in battle. After a few losing skirmishes in which the Corsicans were swiftly overwhelmed by superior numbers of Frenchmen, Ceccaldi withdrew his command into the town of Verdese. Shielded by dense forests and rough terrain, Ceccaldi and an Orezzan native, Colonel Sampiero di Piazzole, were able to hold out in the vales of Orezza, where royalist sentiment was strong. Elsewhere, such as in Ampugnani, many villages capitulated rather than suffer the consequences of resistance, as Villemur did not spare punishment from those who supported the rebels.

With more time, Boissieux might have completed the conquest of the Castagniccia, even with the difficult terrain and fierce defenders of Orezza. Even in the Castagniccia, the faith of common Corsicans in the success of the rebellion was clearly waning, as evidenced by the much feebler response to this new advance than Villemur's invasion of Alesani several months prior. The aforementioned diplomatic developments, however, forced a change of plans. Villemur's attack had been intended to deal a body blow to rebel morale by conquering one of the main centers of royalist support, but King Theodore and most of his generals were not there. The king presently resided with the bulk of his armory in Corti, deep in the interior of the island. Forging a way towards Corti over the mountains between the Castagniccia and the valley of the upper Golo was impractical for such a large force, particularly given local resistance. The only feasible route from the east was the course of the Golo itself, and such an offensive could not be sustained while simultaneously attempting to pacify the Castagniccia; Boissieux did not believe he had the numbers. Even concentrating all his forces on the Golo route might prove difficult given that the terrain prevented him from effectively using his artillery. As Boissieux had been informed of his country's attempts to organize reinforcements, Boissieux decided to hold his position and await this support rather than committing his forces to a counter-guerrilla operation in the Castagniccia which would undoubtedly tie down thousands of men.

Despite continued resistance from Genoa, by early November the French had gathered around two thousand men at Antibes with the intention of shipping them to Corsica, justified to the Genoese Senate as reinforcements to existing battalions which had been depleted over the past year. Deployment, however, was delayed not only by diplomatic wrangling with the Genoese but by the Corsican weather, which had begun to turn foul. Around half the force was able to land at San Fiorenzo on the 6th, but several ships were delayed or damaged by weather, and it appears that not all forces were in Corsica or prepared for battle by the time of Boissieux's advance.

With these reinforcements, Boissieux's corps amounted to at least 4,500 soldiers, of which two battalions (nominally 1,100, but undoubtedly much less than this) would remain in Castagniccia under Villemur's command. Another separate body of troops, estimated at 1,200 men, were under the command of Brigadier Montmorency in Pietralba. The force that Boissieux would lead personally was thus at least 3,500 strong, larger than any army the rebels had ever fielded even at their territorial height. It was true that the French were without their artillery and severely constrained in their use of cavalry owing to the terrain, but this must have been small comfort to the royalists who stood in their path.

As Boissieux prepared for his decisive attack, France managed to score a remarkable own goal on the diplomatic front. Campredon's successor as French minister to Genoa, after a few months of the local consul serving in an interim fashion, was Francois Chaillou de Jonville, who had taken his position in June. In early November, Jonville received a proposal from Versailles which he was instructed to float in an informal fashion among some of the leading Genoese senators. Although not particularly pleased with Boissieux of late, the French government had heeded his long-standing opinion (echoed by Campredon) that the greatest impediment to victory in Corsica was Genoese policy. After all, had not the rebels warmly welcomed the French and turned against them only when it became clear that they brought only a resumption of Genoese rule? The proposal which Jonville received thus suggested nothing less than a complete transfer of administrative and military authority over Corsica from the Genoese Republic to France. France would take all responsibility for justice and tax collection, and all Genoese forces would leave the island. After a set period of years, during which the island would become completely pacified, France would return Corsica to full Genoese rule.

From the French perspective this proposal was win-win. The absence of the Genoese from Corsica would take the wind out of the sails of the rebellion, bringing the war to a speedy conclusion. The bilateral agreement would trump the Fontainebleau accord, thus allowing France to guarantee the exclusion of both Austrian troops and British influence. The French also believed it was good for the Genoese: While Genoa would still be required to pay for the upkeep of French troops, this would be at least partially defrayed by Corsican tax revenue (collected directly by the French), the end of the rebellion would allow the French to maintain a lighter (and thus cheaper) footprint, and the Genoese would also be saved the expense of maintaining their own forces on the island.

For all its apparent virtues, the “Jonville Proposal” went over like a lead balloon in Genoa. The commissioners which Jonville shared the plan with found it too toxic to even discuss, instead telling Jonville that it was outside their jurisdiction or that more clarification was needed. The proposal seemed almost perfectly tailored to confirm the Senate’s fears of a French takeover: Already France had occupied much of the island, demanded payment for ever more troops, and interfered with Genoese governance; now they demanded full administrative control and the expulsion of Genoese troops from the Republic’s own territory, leaving Genoa with a mere nominal sovereignty to be redeemed at some later date—if the French deigned to allow it. At length the plan was submitted to the Grand Council, which reviewed it in secret. Subsequently, the French government was politely informed by the Genoese ambassador, Agostino Lomellini, that the council could not come to an agreement on the matter, which may have been a "diplomatic" way of telling Paris that it was utterly intolerable. The plan was very purposefully never presented to the Senate at large. Nevertheless, the Jonville Proposal hardened the attitudes of key Genoese decision-makers towards France and caused them to look more eagerly at the prospect of imperial assistance.[B]

Footnotes
[1] The British were hardly immune to conspiracy theorizing on the subject. At the same time that Paris was imagining a Vienna-Florence-London axis moving against them in Corsica, English papers were rife with rumors that Boissieux had given an ultimatum to Genoa demanding the cession of Corsica to France. How seriously the government took such tales is unclear, although Genoese representatives in London were warned again that the British government would not tolerate the sale or transferal of Corsica to France.

Timeline Notes
[A] The "War of Jenkins' Ear," which ITTL begins just as IOTL. The British purposefully tried to avoid drawing France into this conflict, which suggests to me that they would be even less likely to overtly meddle in Corsica after October 1739 than in the years leading up to it. Giving the emperor a few diplomatic nudges, however, does not seem unreasonable, and IOTL they do seem to have expressed their concerns about French domination of Corsica to the Austrians despite the ongoing war with Spain.
[B] A real proposal which was really made. It might have worked, too, although with the outbreak of the WoAS IOTL it's unclear if France would have been able or willing to keep up its end of the deal and maintain a presence on the island throughout the war, particularly once the island was surrounded by the British Navy.