Promoted to lieutenant on 1 June 1805, Somerset accompanied Sir Arthur Paget on his visit to Sultan Selim III of the Ottoman Empire, who had been aligning himself too closely with France, in 1807. He became a captain in the 43rd Regiment of Foot on 5 May 1808 shortly before his appointment as aide-de-camp to Sir Arthur Wellesley in July 1808. Somerset accompanied Wellesley's Army when it was sent to Portugal later that month. Somerset fought at the Second Battle of Porto in May 1809, the Battle of Talavera in July 1809 and the Battle of Bussaco (where he was wounded) in September 1810. He was appointed acting military secretary to Wellington in November 1810 and fought with him at the Battle of Pombal in March 1811, the Battle of Sabugal in April 1811 and the Battle of Fuentes de Oñoro in May 1811. Promoted to brevet major on 9 June 1811, he also took part in the Battle of El Bodón in September 1811. He specially distinguished himself at the storming of Badajoz in March 1812 by being the first to mount the breach and by helping to secure the surrender of the French Governor and was duly promoted to lieutenant colonel on 27 April 1812.