Complex sentence: The marriage was often contentious in its early years due to personality conflicts and to what Varina saw as the domination of her husband by his brother, and the relationship was further strained by separations as Jefferson gave campaign speeches and served in the Mexican American War.
Split into simpler sentences: The marriage was often contentious in its early years due to personality conflicts and to problems with each other's families. The relationship was further strained by long periods of separation, first as Jefferson gave campaign speeches and later as he served in the Mexican American War.