The Fugitive Slave Act stated that an escaped slave apprehended in a free state would be returned to his master. A former Act in the 1790s had stated this, but was nullified later. The new Act came about as a part of the 1850 Compromise -- a final effort to avert the growing crisis between slave and free state factions in the government.
Anti-slavery proponents in the North disagreed with this new Act and often refused to enforce it, even though this new, stronger Act imposed fines on officials who refused to cooperate. Southern slave holders felt unduly deprived of what was considered (in legal terms of the time) their property, as some regions of the North encouraged escaped slaves to gather in their communities by this lack of enforcement. But because the new Act held that slaves had no legal rights to trial, a slaveholder had only to produce an affidavit to "prove" someone was an escaped slave. This led to a number of free blacks being kidnapped and sold into slavery.