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 statement highlights the perception in the South that Northern abolitionists were infringing upon their states' rights, particularly the right to maintain the institution of slavery. This belief fueled tensions between the North and South and was a significant factor leading to the secession of Southern states and the subsequent Civil War. The South's commitment to states' rights was often framed in the context of defending slavery, which they viewed as a fundamental aspect of their economic and social system.