In June 1812 the first battalion of the 38th embarked for the Iberian Peninsula, with Willshire commanding the light company. It joined the Royal Regiment and the 9th Foot in the 5th (Leith's) division three days before the Battle of Salamanca, where Willshire was twice wounded. He commanded the light companies of the brigade in the action on the Carrion on 25 October during the retreat from Burgos. In 1813 the division formed part of Graham's corps at the Vittoria and the Siege of San Sebastian. There the 38th was assigned the lesser breach in the first assault and was held in reserve in the second assault, although soon brought up in support of the stormers. He commanded the light companies of the brigade at the passage of the Bidasoa, the Battle of the Nive (9–11 December) and the repulse of the sortie from Bayonne (14 April 1814). He received a brevet lieutenant-colonelcy, and afterwards the Peninsular silver medal with seven clasps.