Slavery became more entrenched in the American South during the late 18th and early 19th centuries as laws defined slaves as property and cotton production expanded, causing the slave population to rapidly increase from 700,000 to 3.5 million. This growing economic and political reliance on slavery isolated the South from the North and caused tensions between states that allowed slavery and those that were moving towards emancipation. However, some like Harriet Tubman fought against slavery by helping slaves escape via the Underground Railroad and inspiring resistance through songs and messages of freedom.