Final answer:
The Union victory in the Civil War and the ratification of the Thirteenth Amendment were the most significant factors that contributed to the end of slavery in the United States.
Step-by-step explanation:
The factor that contributed most directly to end slavery in the United States was the Union victory in the Civil War and the ratification of the Thirteenth Amendment.
The Civil War, fought between the Union (Northern states) and the Confederacy (Southern states), was largely rooted in the issue of slavery. The Union's victory over the Confederacy ultimately led to the emancipation of enslaved African Americans.
Furthermore, with the ratification of the Thirteenth Amendment in 1865, slavery was officially abolished in the United States, making it illegal to own slaves or to involuntarily force individuals into servitude.
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