The Snare by Rafael Sabatini is one of those books that takes a lot of patience at the start, but once you get past the preliminary setup the rest of the story is totally worth the wait.
Set in Portugal, 1810, the story begins with a lengthy account of how a Lieutenant Richard Butler of the British Expeditionary Force gets drunk and then invades a nunnery (mistaking it for a monastery famous for its wine production). As if that breach of etiquette and diplomacy weren't enough, Butler makes it worse by running away...effectively deserting his regiment and becoming an outlaw. Upon learning of the "Tavora Affair," Butler's superiors investigate, and because they cannot find Butler, assume he has been killed by the mob of Portuguese peasants that gathered to protect their convent.
Richard Butler is not the protagonist of the book.
The narrative then shifts to Sir Terence O'Moy, the Adjutant-General in Lisbon. Butler happens to be his brother-in-law, but fortunately for O'Moy's reputation no one knows this detail. Despite being a selfish, stupid, and frivolous person, O'Moy loves his wife Lady Una so much so that he's covered for her brother plenty of times in the past just to protect her feelings.