Which ended slavery in the United States?

Before the American Civil War, Abraham Lincoln and other leaders of the anti-slavery Republican Party sought not to abolish slavery but merely to stop its extension into new territories and states in the American West. This policy was unacceptable to most Southern politicians, who believed that the growth of free states would turn the U.S. power structure irrevocably against them. In November 1860, Lincoln’s election as president signaled the secession of seven Southern states and the formation of the Confederate States of America. Shortly after his inauguration in 1861, the Civil War began. Four more Southern states joined the Confederacy, while four border slave states in the upper South remained in the Union.

Explanation:

roosevelt believed that in light of the country's recent military successes, it was unnecessary to use force to achieve foreign policy goals, so long as the military could threaten force. this belief, and his strategy of “speaking softly and carrying a big stick,” shaped much of roosevelt's foreign policy.