What Mercer came from Europe to fight in American Revolution on side of revolutionaries?

Question

He was either a captain or a major.

Answer 1

That would be Hugh Mercer.

Hugh Mercer (January 17, 1726 – January 12, 1777) was a physician, a brigadier general in the Continental Army and a close friend to George Washington. Mercer died as a result of his wounds received at the Battle of Princeton and became a fallen hero and rallying symbol of the American Revolution.