Tony
Stay serious, Dupont did sign this convention in the believe (maybe naive) - that the French troops were brought back to France.
He was duped, that is not his fault.
And again - the failure of the campaign was not due to Dupont but due to the rigid obeying system in Napoleon's army which did not let any subcommander any initiative. So it is not the servant to blame but the master.
A good memoire to start is :
Tagebuch eines Rheinbundoffiziers aus dem Feldzug gegen Spanien und während spanischer und englischer Kriegsgefangenschaft 1808 - 1814. Wiesbaden 1900