In 1850, Congress strengthened the Fugitive Slave Act, which included criminalizing anyone who aided or abetted an escaping slave. This sent shock waves through the abolitionist community, which felt it had a moral right to help fleeing slaves on northern territory to escape to Canada. Northern law enforcement was also required to help track and return escaped slaves to their masters, no matter what their moral issues with slavery might be.

Many people who had complacently believed slavery was in the process of withering away and dying a natural death in the South were galvanized into action by this law and became more fervent in their insistence that slavery end now. The law, in other words, had a polarizing effect on the country.

One of the people motivated to act was Harriet Beecher Stowe, who wrote Uncle Tom's Cabin in protest against slavery. This book, a runaway bestseller, brought home the plight of slaves in vivid terms and started a firestorm of protest, with many northerners demanding that slavery be immediately abolished.

Through Stowe, and the writings and speeches of escaped slaves, many (though hardly all) Americans began to understand that slavery was not a benign institution in which slaves were carefully taken care of as if they were members of an extended family. As the abuses began to become clearer—young children sold away from their mothers, along with rapes, beatings, and even death dealt to slaves—more and more people became determined that change could not wait.