Waterloo by Bernard Cornwell

Waterloo (Sharpe's Adventures, No. 11) - The English gentry have come to Belgium because they believe that the French have been completely defeated. Among these is Jane Sharpe along with her lover, Johnny Rossendale. Sharpe is aware of their liason and when he meets him, Sharpe declares his terms of divorce to Rossendale. Napoleon has returned to France and is invading Belgium.