Question

In 1860, the institution of slavery was firmly entrenched in the United States; by 1865, it was dead. How did this happen? How and why did Union policy toward slavery and enslaved people change over the course of the war?

Answer 1

The reason was that the tensions, and the subsequent victory of the Union Side in the Civil War allowed the government of Abraham Lincoln to end slavery in the United States.

In other words, he had enough political capital because he had the backing of both union politicians and the people due to the need of leading the war against the Confederate Side in the Civil War.