Sidney played a brilliant part in the military/literary/courtly life common to the young nobles of the time. Both his family heritage and his personal experience (he was in Walsingham's house in Paris during the Saint Bartholomew's Day Massacre), confirmed him as a keenly militant Protestant.
In the 1570s, he persuaded John Casimir to consider proposals for a united Protestant effort against the Catholic Church and Spain. In the winter of 1575-76 he fought in Ireland while his father was Lord Deputy there.[2] In the early 1580s, he argued fruitlessly for an assault on Spain itself. Promoted General of Horse in 1583,[2] his enthusiasm for the Protestant struggle was given free rein when he was appointed governor of Flushing in the Netherlands in 1585. Whilst in the Netherlands, he consistently urged boldness on his superior, his uncle the Earl of Leicester. He carried out a successful raid on Spanish forces near Axel in July 1586.
