General Jean-Charles Pichegru' defection to the Royalist cause appears to have been down to disillusion with the republican movement, coupled with a hefty financial incentive. Pichegru, though, favoured a constitutional monarchy rather than a return to an absolutist one.
Pichegru had made clear his shift away from the Jacobins when he aligned himself with those associated with the Thermidorian Reaction which led to the execution of Robespierre and 21 'of his closest associates' on the 28th July 1794. However, his politics started wavering (mostly in private) long before then according to the English politician and 'spymaster' William Wickham. Michael Durey, in William Wickham, Master Spy, says that while Wickham was working undercover in Switzerland in 1792,
he had heard the rumour that Pichegru, then a corps commander, was
unhappy with the overthrow of the monarchy and the establishment of a
Wickham was working with Louis Joseph, Prince of Condé in seeking to restore the Bourbon monarchy and, unsurprisingly, thought Pichegru would be a big catch for the cause. It was also reported that Pichegru
does not always conceal his dislike of the system he acts under
Although Pichegru had been declared 'Sauveur de la Patrie' for his part in putting down the Insurrection of 12 Germinal, Year III in Paris on the 1st April 1795,
Unbeknown to his host of Parisian admirers, Pichegru had been been
flirting with treason against the Republic for more than a year. In
February 1794, while commander of the Army of the North, he had been
in contact with Colonel Victor Roland, military representative of the
canton of Berne.
Roland, who was working for Wickham, got on well with Pichegru - they were both from the Franche-Comte region in eastern France. Through Roland and from information from a captured French officer, Wickham got information that it might be possible to bribe Pichegru and other French officers. As Durey says, from here the plot gets complicated, but the point has already been established: Pichegru had not been committed to the revolution for quite some time and, (in Feb 1794 or earlier) in his own hand, had written that he had
sympathy for the Constitutional Monarchists in France
Further, even before putting down the Insurrection of 1st April 1795,
Pichegru's belief that the Convention did not have sufficient popular support to sustain a stable republican government without continued military assistance had strengthened.
He was also discouraged by the state of the army, described here (ref: January 1795) as
destitute of everything except courage
On top of these political and military concerns, Pichegru was also was also strapped for cash personally. Thus, Durey concludes,
Saddled with at least one, and possibly two, expensive mistresses,
Pichegru was ripe for anti-republican offers.
According to the French Wikipedia article on Pichegru, these inducements were
un million au comptant, une rente de 200 000 francs, le maréchalat, le
gouvernement d’Alsace et le château de Chambord
Translation: a million in cash, a pension of 200,000 francs, the rank of marshal, the government of Alsace and the castle of Chambord
The Republican government found out, but the still-popular general was merely packed off to Sweden as ambassador. In 1797, he was arrested for plotting and was deported, but escaped. Finally, in 1804 Pichegru was found strangled in prison after having again been arrested for his part in a plot to overthrow Napoleon (see the Pichegru Conspiracy).