States that remained in the union but allowed slavery were referred to as

Border States

Explanation:

The Border States were states that did not declare secession from the Union during the civil war period, but remained slaves states. These states are also called border states because the north of them bordered with free states of the Union and the south bordered with Confederate slave states. The five border states were Delaware, Maryland, Kentucky, Missouri and West Virginia.

the correct answer is:   the domino theory states that if one country in a region came under the influence of communism, then the surrounding countries would follow in a domino effect.