Guy Fawkes served Catholic Spain before joining the Gunpowder Plot.
After converting to Catholicism, Fawkes left England and travelled to the European continent. He fought as a soldier for Catholic Spain in the Eighty Years’ War against the Dutch Protestant forces. During this service he used the name Guido Fawkes and developed military experience, including knowledge relevant to explosives and siege warfare.
That experience made him useful to Robert Catesby, who recruited him for the 1605 conspiracy. Fawkes was chosen to manage the gunpowder placed beneath Parliament because he had the practical skills needed to light and maintain the explosives. He was arrested before he could carry out that task.
Spain is sometimes confused with the country where the plotters hoped to find wider Catholic support. The conspiracy itself was organised in England, while Fawkes’s military service in the Spanish Netherlands occurred years earlier.