The Constitution reared up from a foundation of compromises. Among these compromises was the toleration of slavery by states where it had little to no importance. Article IV, Section 2, Clause 3 of the Constitution required the return of any fugitive from “service or labor” to her/his master from another state into which s/he had fled. In sum, the Union mattered more than did slavery or the enslaved people. A law, the Fugitive Slave Act of 1793, defined the legal mechanisms for returning fugitives. However, as opposition to slavery increased in the North, local governments and private citizens often refused to co-operate or even obstructed the slave-catchers operating among them. Therefore, another compromise, the Compromise of 1850, introduced a much more rigorous Fugitive Slave Act. The new Act further inflamed Northern opinion.
Northern opinion divided more than this brief sketch suggests. Anti-Black racism ran neck and neck with abolitionism in many places. Many parts of the North valued their economic connections to the South and to slavery. Competition between political parties sometimes diverged from principled stands on issues. All these forces came together in New York City before the Civil War.
The city’s government dangled as a puppet of Tammany Hall, the Democratic Party organization. Tammany pols played on the hostility to Blacks felt by the (predominantly Irish) immigrants they were organizing to vote early and often. Judges and prosecutors had often crawled out of the same swamp. New York City policemen sometimes moon-lighted as slave-catchers. Businessmen who wanted to accommodate Southern customers turned a blind eye to it all.
Slave-owners would pay rewards for the return of run-aways, so Blacks in New York—people of color in an overwhelmingly White city–were deer in the jacklights of slave-catchers. This hunt only intensified with the passage of the Fugitive Slave Act of 1850, which offered handsome fees to both slave-catchers and the judges who approved their transfer Southward. Lured by the money, the slave-catchers sometimes kidnapped—and judges regularly approved the transfer of—free Blacks who were knowingly misidentified as fugitives. Applying the term not just to New York, but to the whole of the North, one historian has labeled this the “Reverse Underground Railroad.”
Highly publicized stories of free Blacks kidnapped into slavery appalled a growing audience of Northern Whites. Five Black boys were kidnapped from Philadelphia in 1825, then four survivors providentially returned to tell their story of the Black “Trail of Tears” that ran from the Upper South to the new cotton lands of the Southwest. In 1853, Solomon Northup wrote of his “12 Yeas a Slave.” Not for nothing has Elizabeth Varon called her book on the Union troops Armies of Deliverance.
 Banks financed the cotton trade and its spendthrift planters; insurers and ship-owners profited from the massive cotton exports.
 Jonathan Daniel Wells, The Kidnapping Club: Wall Street, Slavery, and Resistance on the Eve of the Civil War (2020), reviewed by Harold Holzer, WSJ, 19 October 2020.
 Richard Bell, Stolen: Five Free Boys Kidnapped into Slavery and Their Astonishing Odyssey (2019), reviewed by David S. Reynolds, WSJ, 17 October 2019. .