Hesse served in the Peninsular War between February 1813 and April 1814, and was present at all the actions where the 18th Light Dragoons were engaged, including Morales on 2 June 1813, Vittoria on 21 June, where he was severely wounded in the wrist, Nivelle on 10 November 1813, the Nive in December, Orthez on 27 February 1814, Croix d'Orade on 8 April, and Toulouse on 10 April. In late 1813 negotiations began for Princess Charlotte's marriage to the Hereditary Prince of Orange, and Hesse was requested to return her letters and a portrait, which had been sent out and delivered personally to him by Lord Wellington after the Battle of Vittoria. Hesse was initially reluctant to part with them but eventually relented. Lady Blessington states the letters and portrait were recovered by the Margravine of Ansbach's son Keppel Craven at the instigation of Princess Charlotte herself, while Gronow says Hesse surrendered them to Admiral Lord Keith on the request of the Regent. The letters were still in the possession of the Countess de Flahaut (Keith's daughter) in 1845.