The Snare by Rafael Sabatini, Fiction, Action & Adventure

The problem was traitors in high places: traitors secretly opposing his methods and playing the spy for the enemy. All depended on secrecy and unity of action. And then a young British officer made a moronic blunder, and gave the plotters their chance to upset the delicate balance. Their influence caused the Portuguese Council of Regency to demand that the culprit be made a scapegoat. He was at large -- and the man responsible for his capture and execution was his own brother-in-law, Sir Terrence Amoy. Amoy, British adjutant-general at Lisbon.