Final answer:
The U.S. Marshals' early law enforcement duties included enforcing tax laws, implementing the Fugitive Slave Law of 1850, and upholding various civil rights laws during Reconstruction.
Step-by-step explanation:
One of the U.S. Marshals' first law enforcement duties was to enforce tax laws and serve court documents, as seen in early incidents such as the confrontation with John Fries and his militia in 1799 over tax issues. Additionally, U.S. Marshals were instrumental in upholding the Fugitive Slave Law of 1850, which included the duty to capture fugitive slaves and return them to their claimants. This law imposed heavy consequences on those who did not assist, as well as on the accused who were denied fair trials and the right to self-defense. Later on, during the Reconstruction era, the U.S. Marshals along with other federal forces were active in enforcing civil rights laws like the First Enforcement Act of 1870, prosecuting individuals under the Ku Klux Klan Act, and ensuring the right to vote for newly enfranchised African Americans.