In early April 1835 the Duke, as Foreign Secretary, sent Lord Eliot on a mission to Spain and asked Gurwood to accompany him. The two men were chosen for their fluent French and Spanish and for their understanding of the country and its politics. The first Carlist War had broken out in 1833 on the death of Ferdinand VII and the British Government, which did not take sides, was horrified by the summary execution of prisoners by both armies. The mission managed to make contact with both sides including with Tomas di Zamalcarregui, the Basque general on the Carlist faction with which Gurwood was more in political sympathy. They secured a signed Convention on the fair treatment of prisoners and the wounded but, sadly, this lasted only a few months.