The following correspondence was conducted in the wake of Murat’s defection to the Allies in 1814, as Napoleon attempted to bring his errant brother-in-law back into the fold before it was too late.
It is worth noting that Joseph Bonaparte declines, in his letter to Murat, to “point out to him the iniquity of his conduct,” “frankly” or otherwise, and opts for a gentler appeal to Murat’s character and patriotism instead.
Source: The Confidential Correspondence of Napoleon Bonaparte with His Brother Joseph: Selected and Translated, with Explanatory Notes, from the ‘Mémoires Du Roi Joseph,’ Volume 2. New York (D. Appleton and Company), 1856. Pages 321-323.