At the outbreak of the Civil War in 1861, pro-Union sentiment was strong in western Virginia, eastern Tennessee, and western North Carolina primarily because?

There were relatively few slaves or large plantations in these regions

Explanation:

The Civil War in the United States began in 1861, after decades of simmering tensions between northern and southern states over slavery, states' rights and westward expansion.

Growing abolitionist sentiment in the North after the 1830s and northern opposition to slavery's extension into the new western territories led many southerners to fear that the existence of slavery in America and thus the backbone of their economy was in danger.

On September 17, the Army of the Potomac hit Lee's forces (reinforced by Jackson's) in what became the war's bloodiest single day of fighting. Total casualties at the Battle of Antietam (also known as the Battle of Sharpsburg) numbered 12,410 of some 69,000 troops on the Union side, and 13,724 of around 52,000 for the Confederates.

Fewer slaves and Fewer large plantations were found in these areas

Explanation:

the outbreak of the American civil war in 1861 was majorly because of the unending disagreements between the Union of the North and the confederate states of south over slavery, state rights and the expansion westward.

The Union was strongly against slavery and the expansion of slavery into new territories as proposed by the south. the south found this opposition worrying because they have built their economy on the use of slave labor to grow agricultural produce for commercial purpose. the continues disagreements finally led to the American civil war in 1861.

pro - union sentiment was strong in Western Virginia, eastern Tennessee and Western North Carolina because there were few slaves and smaller plantations in the regions.