Complex sentence: It was the first field army-level engagement in the Eastern Theater of the American Civil War to take place on Union soil and is the bloodiest single-day battle in American history, with a combined tally of 22,717 dead, wounded, or missing.
Split into simpler sentences: It was the first Field army-level engagement in the Eastern Theater of the American Civil War to take place on Union soil. It is the bloodiest single-day battle in American history, with a combined tally of 22,717 dead, wounded, or missing.