Complex sentence: He promised to marry her after the wars with Napoleon were over; however, his mother, Countess Natasha Rostova wished for him to marry Princess Marya Bolkonskaya due to her being a wealthy heiress, which would save the family's failing finances.
Split into simpler sentences: He promised to marry her after the wars with Napoleon were over. However, his mother, Countess Natasha Rostova wished for him to marry Princess Marya Bolkonskaya due to her being a wealthy heiress, which would save the family's failing finances.