"had become involved in a duel"
A duel was a formal engagement between two gentlemen to settle a dispute by single combat. Duels were fought over matters of honour, and would come about if one man offended the other or his family. Famously scheduled to take place at dawn, duels usually had very fixed rules and the expected outcome for the losing party was death.
Towards the end of the 18th Century, duelling fell out of favour in the USA and gradually declined.