The first major battle and bloodiest one-day battle during the Civil War. O Chickamauga
O Antietam
O Gettysburg

Antietam

Explanation:

Beginning early on the morning of September 17, 1862, Confederate and Union troops in the Civil War clash near Maryland's Antietam Creek in the bloodiest single day in American military history. The Battle of Antietam marked the culmination of Confederate General Robert E. Lee's first invasion of the Northern states.

a. the moon was locked into orbit by the earths gravity as it was passing by and capture hypothesis i think