There were 24 states which remained loyal to the Union during the war:
California, Connecticut, Illinois, Indiana, Iowa, Kansas, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, Ohio, Oregon, Pennsylvania, Rhode Island, Vermont, and Wisconsin.
The Union counted Virginia as well, and added Nevada and West Virginia. It added Tennessee, Louisiana, and other rebel states as soon as they were reconquered.
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The states that remained part of the United States and fought the Confederacy during the Civil War.
The Border States were a key part of the US Civil War. They were slave states that did not join the Confederacy, it was important for the Union to prevent that they did not secede and join the South.
Jamaica was still part of the British Empire during the American Civil War. So a loose answer is that the would have helped the Union.
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Virginia, North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, Texas, Arkansas, and Tennessee seceded from the Union. Missouri, Kentucky, Maryland, and Delaware were slave states that remained in the Union.