Complex sentence: Although this may seem strange, given that the majority of white Southerners did not own slaves, virtually every white Southerner supported slavery because he did not want to be at the bottom of the social ladder.
Split into simpler sentences: Although this may seem strange, given that the majority of white Southerners did not own slaves, most white Southerners supported slavery. It has been supposed that this is because they did not want to be at the bottom of the social ladder.