The statement that best shows how the issue of states' rights contributed to the onset of the Civil War is:
Many Southerners believed the rights of their states were threatened by Northern abolitionists.
This encapsulates the central conflict over states' rights during the Civil War era, particularly as it relates to the Southern states' desire to maintain the institution of slavery against what they perceived as the encroaching influence of Northern abolitionist sentiments that sought to limit or eliminate it.