Increasing animosity between Southerners and Northerners over slavery helped bring on the Civil War, and the Underground Railroad’s work aiding enslaved people fleeing to the free North and Canada became a major source of conflict. Near Preston, two major stations operated in Maryland: one run by local Quakers who long opposed slavery, and another run by Harriet Tubman and her parents, Benjamin and Harriet Ross. The success of these and other Maryland stations led slaveholders to call local and statewide conventions denouncing the North for harboring fugitive slaves. When the two Preston-area stations were exposed in 1857-58, Maryland slave owners were further enraged, though most operators, including Tubman, escaped to Northern states and Canada. Northern abolitionists were also angered when the captured conductor Sam Green received a ten-year prison sentence. Heightened by the Underground Railroad, these opposing resentments ultimately found expression in armed conflict in 1861.