Which statement illustrates the MOST controversial aspect of the notion of Manifest Destiny as promoted by President James K. Polk? A. Many Americans feared that an aggressive push westward would dangerously stretch the nation's resources. B. The idea that Native American lands should be respected and preserved from further encroachment. C. Military leaders feared that any push westward would cause foreign powers anticipating territorial disputes and possibly war, to build up their military forces. D. Many northern citizens recognized the commercial potential of the push westward, but feared it could also upset the slave and free-state balance and lead to permanent acceptance of slavery.