Final answer:
The first major battle on Union soil, the bloodiest day of the Civil War, and an important border state location was Antietam.
Step-by-step explanation:
The battle that matches all the descriptions given is Antietam. Antietam was fought on Union soil and took place in Maryland, which was a border state during the Civil War. It was considered the bloodiest day of the war, with approximately 23,000 casualties. The battle is significant because it provided a turning point for the Union and led to President Lincoln's issuance of the Emancipation Proclamation.
Learn more about Antietam, a major battle during the Civil War