Final answer:
The Fugitive Slave Act of 1850 required Americans to return runaway slaves to their owners, making it a crime to help or harbor them.
Step-by-step explanation:
The action by Congress that required Americans to return runaway slaves to their owners was the passage of the Fugitive Slave Act of 1850.
This law made it a crime to help or harbor runaway slaves and required the cooperation of all citizens in capturing and returning escaped slaves. It was part of the Compromise of 1850, which aimed to ease tensions between the North and South over the issue of slavery.
Under the Fugitive Slave Act, individuals who were suspected of being runaway slaves could be arrested and returned to their owners, even if they were living in free states. This law further inflamed abolitionist sentiment and contributed to the growing divide between North and South leading up to the American Civil War.
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